<h3>Objective:</h3> To describe clinical characteristics, findings and progression among patients with Mills’ Disease. <h3>Background:</h3> Mills’ disease is a rare condition characterized by slowly progressive spastic hemiparesis. Typically, there are no prominent lower motor neuron signs but the disease may still be part of amyotrophic lateral sclerosis spectrum (ALS). <h3>Design/Methods:</h3> This is a retrospective chart review study of 12 patients with Mills’ Syndrome who met criteria of gradual unilateral ascending/descending spastic hemiparesis. Progression of symptoms, signs, pulmonary function tests (PFTs) and amyotrophic lateral sclerosis functional-rating scale-revised (ALSFRS-R) score were monitored at initial and last visits. Diagnostic workup and death prevalence were collected. <h3>Results:</h3> 11 patients with Mills had ultimately primary lateral sclerosis (PLS) and 1 patient had ALS diagnosis. All PLS patients developed spasticity within 4 years average from symptom onset. All PLS patients had non-dominant side as initial symptom and none progressed to involve contralateral side. Of PLS patients, 36%, 46% and 18 % had left arm, leg and hemi-side involvement at onset respectively. If symptoms started in the arm, an average of 9-years elapsed before weakness occurs in the leg. If onset was in the leg, an average of 2-years until weakness occurs in the arm. 27% and 18% of PLS patients developed dysarthria and choking within an average of 5 and 7 years of onset respectively. Cervical MRI showed evidence of central canal stenosis in 2 patients. Electromyography showed acute and chronic denervation changes in ALS patient. 50% of PLS patients had worsening PFTs but none required bilevel-positive-airway-pressure (BiPAP), whereas the ALS patient required BiPAP within 3 years of symptom onset. In PLS patients, ALSFRS-R score worsened by 0.5 points/year. None died over 10 year follow-up period. <h3>Conclusions:</h3> ALS Mills’ Disease is rare. PLS Mill’s variant has slower progression if it starts in the nondominant upper-extremity with mild bulbar and respiratory symptoms. <b>Disclosure:</b> Dr. Moussallem has nothing to disclose. Dr. Jawdat has nothing to disclose. Dr. Jabari has nothing to disclose. Dr. Pasnoor has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Argenx BVBA. Dr. Pasnoor has received personal compensation in the range of $500-$4,999 for serving as a Consultant for catalyst. Dr. Pasnoor has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Takeda Pharmaceuticals. Dr. Pasnoor has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Janssen Pharmaceutical. Dr. Pasnoor has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Immunovant. Dr. Pasnoor has received personal compensation in the range of $500-$4,999 for serving as a Consultant for UCB. Dr. Pasnoor has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for catalyst pharmaceuticals. Dr. Pasnoor has received research support from NIH. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for EcoR1 Capital, LLC. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Cello Health BioConsulting, previously Defined Health. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Sanofi. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for CSL Behring. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Octapharma. Dr. Dimachkie has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for ArgenX. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Spark Therapeutics. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for RAF-5. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Houston NeuroCare. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Shire. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Catalyst. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Kezar. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for RaPharma/UCB. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for ArgenX. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Sanofi. An immediate family member of Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Kezar. Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Catalyst. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Takeda. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for SCHOLAR Rock. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Jenssen. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Amazentis. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Spark Therapeutics. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Roivant. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for SANEM. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Medlink. Dr. Dimachkie has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for CSL Behring. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Third Rock. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Orphazyme . Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Astellas. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Kanssen. Dr. Dimachkie has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Roivant . Dr. Dimachkie has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Covance / Labcorp. The institution of Dr. Dimachkie has received research support from CSL Behring. The institution of Dr. Dimachkie has received research support from Orphazyme. The institution of Dr. Dimachkie has received research support from FDA-OOPD. Dr. Dimachkie has received intellectual property interests from a discovery or technology relating to health care. Dr. Dimachkie has received publishing royalties from a publication relating to health care. Dr. Statland has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Arrowhead. Dr. Statland has received personal compensation in the range of $500-$4,999 for serving as a Consultant for ML Bio. Dr. Statland has received personal compensation in the range of $500-$4,999 for serving as a Consultant for MT Pharma. Dr. Statland has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Epic Bio. Dr. Statland has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Armatus . Dr. Statland has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Dyne Therapeutics. Dr. Statland has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Avidity . Dr. Statland has received personal compensation in the range of $5,000-$9,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Fulcrum Therapeutics. Dr. Statland has received personal compensation in the range of $5,000-$9,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Roche. The institution of Dr. Statland has received research support from NIH. The institution of Dr. Statland has received research support from FSHD Society. The institution of Dr. Statland has received research support from Friends of FSH Research. The institution of Dr. Statland has received research support from FSHD Canada. The institution of Dr. Statland has received research support from MDA.