Although patients with obstructive sleep apnea often report falling asleep while driving, the frequency of auto accidents involving these patients has not been rigorously studied. Therefore, we compared the driving records of 29 patients with obstructive sleep apnea with those of 35 subjects without sleep apnea. The patients with sleep apnea had a sevenfold greater rate of automobile accidents than did the subjects without apnea (p less than 0.01). The percentage of persons with one or more accidents was also greater in the patients with apnea than in the control subjects without apnea (31% versus 6%, p less than 0.01). The percentage of persons having one or more accidents in which they were at fault was also greater in the patients with apnea than in the control subjects (24% versus 3%, p less than 0.02). The automobile accident rate of the patients with sleep apnea was 2.6 times the accident rate of all licensed drivers in the state of Virginia (p less than 0.02). In addition, 24% of patients with sleep apnea reported falling asleep at least once per week while driving. We conclude that patients with obstructive sleep apnea have a significantly higher frequency of auto accidents than do subjects without apnea. Impaired drivers with sleep apnea may cause many preventable auto accidents.