To assess the occurrence and related features of motor vehicle crashes in patients with dementia of the Alzheimer type (DAT) we studied 30 patients who had been followed longitudinally and 20 healthy age-matched control subjects. Data were gathered from first-degree relatives who had lived with the subjects for the previous 5 years or more. Forty-seven percent of the DAT patients incurred at least one crash while they were driving, whereas only 10% of the control subjects had had a crash in the previous 5 years. The odds ratio for crashes in the DAT group was 7.9 (p less than 0.01). Moreover, in 77% of DAT patients, a deterioration in driving performance was noted, and 63% of the patients had stopped driving. However, only 42% of the DAT patients who stopped driving did so before a crash occurred. Mean illness duration was 4.0 (+/- 1.8) years, and the mean Mini-Mental Status Examination score was 19.9 (+/- 6.3) at the time of the first crash in the DAT group. The occurrence of crashes was not significantly correlated with dementia severity or with disease duration. These data suggest that the occurrence of driving crashes in patients with DAT is an important public health problem.