Sleep-related motor vehicle accidents are a serious safety hazard both for the driver who falls asleep and for others on the road. Sleep disorders may be significant contributing factor in some of these accidents. We reviewed data on sleep-related accidents from 70 control subjects and 424 adults with four categories of sleep disorders: sleep apnea, narcolepsy, other disorders of excessive sleepiness, and sleep disorders without excessive sleepiness. The proportion of individuals with sleep-related accidents was 1.5-4 times greater in the hypersomnolent patient groups than in the control group. In patients with hypersomnia, the incidence of sleep-related accidents per year of excessive sleepiness was 3-7%. Although the proportion of patients with sleep-related accidents was highest in narcoleptics, apneics were involved in more sleep-related accidents because of their greater number. Apneics and nacroleptics accounted for 71% of all sleep-related accidents. The proportion of severe apneics who had sleep-related accidents was almost twice that of patients with mild or moderate apnea. Mean sleep latency by Multiple Sleep Latency Test did not differ significantly in patients with accidents and those without. Patients with a wide variety of sleep disorders appear to be at increased risk for sleep-related accidents. The severity and duration of hypersomnia are probably not the only factors that contribute to that risk. These findings have implications for the management of patients with sleep disorders.