<h3>Objective:</h3> To assess thalamic structural and functional MRI alterations and investigate their correlations with physical activity (PA) and cardiorespiratory fitness (CRF) levels in progressive MS (PMS) patients. <h3>Background:</h3> Patients with PMS have insufficient levels of PA and CRF, which showed some associations with measures of structural MRI damage. Functional MRI (fMRI) correlates of reduced PA/fitness have never been explored. Given the role of thalamus in motor planning, sensory processing and cognition, abnormal thalamic resting state (RS) functional connectivity (FC) might explain PA/fitness levels in these patients. <h3>Design/Methods:</h3> Ninety-one PMS patients performed a cardiopulmonary exercise test and wore an accelerometer for 7 days to assess PA/CRF levels. They underwent, together with 37 matched healthy controls (HC), a structural and RS fMRI acquisition at 3.0T, used to derive whole-brain and thalamic atrophy and thalamic RS FC. Between-group comparisons of MRI measures and their correlations with PA/CRF variables were assessed. <h3>Results:</h3> PMS patients had whole-brain and subcortical atrophy compared to HC (all p<0.001). Patients showed decreased intra- and inter-thalamic RS FC, decreased RS FC of the thalamus with caudate nucleus, cerebellum and anterior cingulate cortex (ACC), and increased thalamic RS FC with the hippocampus and some occipital regions. Lower CRF correlated with lower white matter volume (r=range 0.28;0.31, p=range 0.003;0.01), decreased thalamic RS FC with the ACC (r=range 0.22;0.28, p=range 0.01;0.04), and increased thalamic RS FC with the hippocampus, calcarine cortex, and lingual gyrus (r=range −0.26; −0.21, p=range 0.01;0.04). Lower PA correlated with decreased inter-thalamic RS FC (r=0.27, p=0.02), and increased thalamic RS FC with the hippocampus (r=−0.3, p=0.01) and lingual gyrus (r=−0.23, p=0.04). <h3>Conclusions:</h3> Only white matter atrophy correlated with CRF. Conversely, abnormal RS FC in the thalamic network showed various maladaptive associations with PA/CRF in PMS patients. Thalamic RS FC might be used to monitor the efficacy of rehabilitative and disease-modifying treatments in PMS patients. <b>Disclosure:</b> Mr. Romanò has nothing to disclose. Maria Assunta Rocca has received personal compensation in the range of $500-$4,999 for serving on a Speakers Bureau for Bayer, Biogen, Bristol Myers Squibb, Celgene, Genzyme, Merck Serono, Novartis, Roche, and Teva. The institution of Maria Assunta Rocca has received research support from Italian Ministry of Health, MS Society of Canada and Fondazione Italiana Sclerosi Multipla. Paola Valsasina has received personal compensation in the range of $500-$4,999 for serving on a Speakers Bureau for ACCMED. Mr. Morozumi has nothing to disclose. Dr. Amato has received personal compensation for serving as an employee of AOUCareggi. Dr. Amato has received personal compensation for serving as an employee of AOUCareggi. Dr. Amato has received personal compensation for serving as an employee of AOUCareggi. Dr. Amato has received personal compensation for serving as an employee of AOU Careggi. Dr. Amato has received personal compensation for serving as an employee of AOUCareggi. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Roche. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Novartis. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Sanofi Genzyme. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Novartis. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Biogen. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Merck. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Roche. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Novartis. Dr. Amato has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Sanofi genzyme. The institution of Dr. Amato has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Celgene BMS. The institution of Dr. Amato has received research support from Merck. The institution of Dr. Amato has received research support from Biogen. The institution of Dr. Amato has received research support from Roche. Giampaolo Brichetto, MD, PhD has received personal compensation for serving as an employee of Italian MS Society. Mr. Bruschi has nothing to disclose. Dr. Chataway has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Janseen. Dr. Chataway has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Biogen. Dr. Chataway has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Merck. Dr. Chataway has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for NerveGen. Dr. Chataway has received personal compensation in the range of $500-$4,999 for serving on a Speakers Bureau for Novartis. Nancy Chiaravalloti has received personal compensation in the range of $100,000-$499,999 for serving as a Director of Neuroscience Research with Kessler Foundation. Dr. Cutter has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Biodelivery Sciences International, Biogen, Click Therapeutics, Genzyme, Genentech, GW Pharmaceuticals, Immunic, Klein-Buendel Incorporated, Medimmune/Viela Bio, Medday, Merck/Serono, Neurogenesis LTD, Novartis, Osmotica Pharmaceuticals, Perception Neurosciences, Recursion/Cerexis Pharmaceuticals, Regeneron, Reckover Pharmaceuticals, Roche, TG Therapeutics.. Dr. Cutter has received personal compensation in the range of $5,000-$9,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Applied Therapeutics, AI therapeutics, AMO Pharma, Astra-Zeneca, Avexis Pharmaceuticals, Biolinerx, Brainstorm Cell Therapeutics, Bristol Meyers Squibb/Celgene, CSL Behring, Galmed Pharmaceuticals, Green Valley Pharma, Horizon Pharmaceuticals, Immunic, Karuna Therapeutics, Mapi Pharmaceuticals LTD, Merck, Mitsubishi Tanabe Pharma Holdings, Opko Biologics,Prothena Biosciences, Novartis, Regeneron, Sanofi-Aventis, Reata Pharmaceuticals, NHLBI (Protocol Review Committee), University of Texas Southwestern, University of Pennsylvania, Visioneering Technologies, Inc.. Dr. Cutter has received personal compensation in the range of $500-$4,999 for serving as an Editor, Associate Editor, or Editorial Advisory Board Member for JASN. Ulrik Dalgas has received personal compensation for serving as an employee of Roche. Ulrik Dalgas has received personal compensation for serving as an employee of Bristol Meyers Squibb. Dr. DeLuca has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Novartis. Dr. DeLuca has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Biogen. Dr. DeLuca has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Celgene. Dr. DeLuca has received personal compensation in the range of $10,000-$49,999 for serving on a Speakers Bureau for Biogen. The institution of Dr. DeLuca has received research support from Biogen. The institution of Dr. Farrell has received personal compensation in the range of $500-$4,999 for serving as a Consultant for GW Pharma / Jazz. The institution of Dr. Farrell has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Merz. Peter Feys has received personal compensation for serving as an employee of UHasselt. The institution of Peter Feys has received research support from fwo flanders . The institution of Peter Feys has received research support from Kbs. The institution of Peter Feys has received research support from Charcot foundation Belgium. Jennifer Freeman has nothing to disclose. Maria Matilde Inglese has nothing to disclose. Ms. Meza has nothing to disclose. Dr. Salter has received personal compensation in the range of $5,000-$9,999 for serving as an Editor, Associate Editor, or Editorial Advisory Board Member for American Heart Association. Dr. Sandroff has nothing to disclose. Anthony Feinstein, MD has received personal compensation in the range of $500-$4,999 for serving on a Speakers Bureau for Novartis, Biogen, Roche and Sanofi-Genzyme. The institution of Anthony Feinstein, MD has received research support from Multiple Sclerosis Society of Canada. Anthony Feinstein, MD has received publishing royalties from a publication relating to health care. Robert Motl has nothing to disclose. Dr. Filippi has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Bayer, Biogen Idec, Merck-Serono, Novartis, Roche, Sanofi Genzyme, Takeda, and Teva Pharmaceutical Industries. Dr. Filippi has received personal compensation in the range of $5,000-$9,999 for serving on a Speakers Bureau for Bayer, Biogen Idec, Merck-Serono, Novartis, Roche, Sanofi Genzyme, Takeda, and Teva Pharmaceutical Industries. Dr. Filippi has received personal compensation in the range of $5,000-$9,999 for serving as an Editor, Associate Editor, or Editorial Advisory Board Member for Springer Nature. The institution of Dr. Filippi has received research support from Biogen Idec, Merck-Serono, Novartis, Roche, Teva Pharmaceutical Industries, Italian Ministry of Health, Fondazione Italiana Sclerosi Multipla, and ARiSLA (Fondazione Italiana di Ricerca per la SLA).