We assessed the influence of the neurological and cognitive impairments of Huntington's disease (HD) on automobile driving. In a group of 73 HD outpatients, 53 (72%) continued to drive after illness onset. Those no longer driving had more severe symptoms than those still driving. Twenty-nine HD patients who were still driving and 16 healthy control subjects underwent a clinical examination, a cognitive examination, a driving-simulator assessment, and completed questionnaires about driving history and habits. HD patients performed significantly worse than control subjects on the driving-simulator tasks and were more likely to have been involved in a collision in the preceding 2 years (58% of HD vs. 11% of control subjects). Patients with collisions were less functionally impaired but had slower simple reaction time scores than did those without collisions. HD patients are at increased risk for accidents, but patients who have accidents are not easily distinguished from those who do not.