To determine whether drivers with sleep apnea are at increased risk of motor vehicle crash; whether disease severity, daytime sleepiness, or both disease severity and daytime sleepiness affect this risk, and whether treatment of sleep apnea reduces crash risk. Systematic review of published literature. N/A. Patients with sleep apnea. N/A. Forty pertinent studies were identified. For studies investigating whether noncommercial drivers with sleep apnea have increased crash rates, the majority (23 of 27 studies and 18 of 19 studies with control groups) found a statistically significant increased risk, with many of the studies finding a 2 to 3 times increased risk. Methodologic quality of the studies did not influence this relationship (p = .22). For commercial drivers, only 1 of 3 studies found an increased crash rate, with this association being weak (odds ratio of 1.3). The evidence was mixed regarding whether the risk of crash involvement is proportional to the severity of the sleep apnea, with about half of the studies finding a statistically significant increased risk with increased severity. Correlation with subjective daytime sleepiness and crash risk was also found in only half of the studies reviewed. Treatment of sleep apnea consistently improved driver performance (including crashes) across all studies. Noncommercial drivers with sleep apnea are at a statistically significant increased risk of involvement in motor vehicle crashes. Studies did not consistently find that daytime sleepiness and the severity of sleep apnea were correlated with crash risk. Successful treatment of sleep apnea improves driver performance. Clinicians should educate their patients with sleep apnea about the importance of treatment adherence for driving safety.