<h3>Objective:</h3> To assess correlates of fluctuations in moderate-advanced dementia with Lewy bodies [DLB]. <h3>Background:</h3> Cognitive fluctuations (spontaneous episodic deficits in attention, arousal, cognition, and behavior) are a core DLB feature, but remain poorly understood. <h3>Design/Methods:</h3> The PACE-DLB study collected measures of Lewy body disease, demographics, and patient and caregiver experiences from moderate-advanced DLB individual-caregiver dyads. Fluctuations were assessed via the Mayo Fluctuations Scale (MFS, range 0–4) and Clinician Assessment of Fluctuation (CAF, range 0–16). Concordance was assessed using Spearman correlation for raw scores and Cohen’s Kappa for assessing scores above/below the cut-off. Nonparametric methods including Spearman correlation, two-sample Wilcoxon test, and odds ratios assessed factors associated with fluctuations. The FDR method corrected raw scores for multiple testing. <h3>Results:</h3> Patients (n=137) identified as white non-Hispanic (94.9%) and male (79.6%); mean age 75.0 (SD 8.0), disease duration 3.0 years (SD 2.4). Caregivers were typically wives (72.3%) and daughters (13.1%). On the MFS, 116 participants (84.7%) had fluctuations (score ≥3) (mean 3.3, SD 0.8) versus 116 (84.7%) on the CAF (score >0) (correlation: 0.2, p<0.001; fluctuations absent/present: 0.3, p=0.002). The mean CAF score was 8.8 (SD 6.6). Variables associated with a positive MFS included higher sleepiness scores (p=0.02), lower QoL-AD scores (p=0.02), and parkinsonism (p=0.02). No variables associated with a positive CAF. Higher MFS scores associated with higher sleepiness scores (p<0.001), lower QoL-AD, and more autonomic symptoms (p=0.03). Higher CAF scores associated with higher Neuropsychiatric Inventory severity scores (p<0.0001), hallucinations (p=0.045), orthostatic hypotension (p=0.02), more comorbidities (p=0.04), and higher sleepiness scores (p=0.045). Fluctuations associated with worse patient and caregiver quality of life and higher caregiver burden and depression. <h3>Conclusions:</h3> The MFS and CAF correlated weakly. Fluctuations associated with sleepiness, autonomic symptoms, parkinsonism, and neuropsychiatric symptoms. Given the impact of fluctuations on quality of life and caregiver burden and depression, fluctuations are an important target for future pharmacologic therapies. <b>Disclosure:</b> Dr. Tholanikunnel has nothing to disclose. Mr. LaBarre has nothing to disclose. Ms. Sovich has nothing to disclose. Li Zhigang has nothing to disclose. James Galvin has nothing to disclose. Dr. Paulson has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Exicure. The institution of Dr. Paulson has received research support from National Institutes of Health. Dr. Paulson has a non-compensated relationship as a Member, Nominating committee with American Neurological Association that is relevant to AAN interests or activities. An immediate family member of Dr. Maixner has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Law office . The institution of Dr. Manning has received research support from Department of Defense. The institution of Dr. Fields has received research support from National Institutes of Health. The institution of Dr. Fields has received research support from Patient-Centered Outcomes Research Institute. Ms. Lunde has received personal compensation in the range of $500-$4,999 for serving as an Editor, Associate Editor, or Editorial Advisory Board Member for Mayo Clinic . Dr. Boeve has received personal compensation in the range of $10,000-$49,999 for serving as an officer or member of the Board of Directors for Rainwater Charitable Foundation. The institution of Dr. Boeve has received research support from Alector. The institution of Dr. Boeve has received research support from GE Healthcare. The institution of Dr. Boeve has received research support from Transposon. The institution of Dr. Boeve has received research support from Cognition Therapeutics. Dr. Boeve has received publishing royalties from a publication relating to health care. Ms. Forsberg has nothing to disclose. Angela Taylor has received personal compensation for serving as an employee of Lewy Body Dementia Association . The institution of Dr. Armstrong has received research support from National Institute of Aging. The institution of Dr. Armstrong has received research support from Florida Department of Health. The institution of Dr. Armstrong has received research support from Lewy Body Dementia Association. Dr. Armstrong has received research support from Parkinson Foundation. Dr. Armstrong has received personal compensation in the range of $5,000-$9,999 for serving as a DSMB member with Alzheimer’s Clinical Trials Consortium. Dr. Armstrong has received personal compensation in the range of $5,000-$9,999 for serving as a DSMB member with Alzheimer’s Disease Cooperative Study. Dr. Armstrong has a non-compensated relationship as a Member, Scientific Advisory Council with Lewy Body Dementia Association that is relevant to AAN interests or activities.