© 2014 American Congress of Rehabilitation Medicine. Objective To examine the relationship between third ventricular width, a measure of thalamic brain atrophy, and motor vehicle violation type and frequency in a cohort of patients with multiple sclerosis (MS).Design Retrospective cohort study.Setting Tertiary care university hospital. Participants Thirty-five individuals with clinically confirmed relapsing-remitting multiple sclerosis and 35 age-, sex-, and education-matched community-dwelling healthy comparisons (N=70).Participants were aged between 25 and 65 years.Interventions Not applicable.Main Outcome Measures Data on motor vehicle violations were obtained from an online database (Iowa Courts Online). The violations were categorized as follows: (1) speeding, (2) nonmoving safety, (3) administrative, (4) alcohol-related offense, (5) moving safety, and (6) total violations. Neuropsychological performance in all major cognitive domains was obtained. Thalamic atrophy for the patients with MS was determined via third ventricular width measurement.Results The MS group had a greater number of overall violations, administrative violations, and nonmoving safety violations. The groups differed on neuropsychological tasks measuring visuospatial skills, speeded language, learning, and executive functioning, after controlling for affective symptoms. Third ventricular width was associated with total violations as well as moving safety violations. Finally, third ventricular width accounted for a significant variance in driving violation frequency above and beyond demographic variables and neuropsychological factors.Conclusions There is an increased frequency of motor vehicle violations among patients with multiple sclerosis, and the number of violations can be predicted by thalamic brain atrophy.