We studied 50 consecutive patients to test the hypothesis that successful treatment of obstructive sleep apnea with nasal continuous positive airway pressure (nasal CPAP) will decrease automobile accidents in patients with sleep apnea. Thirty-six (72%) of the patients reported using nasal CPAP regularly during 2 yr. Fourteen patients reported they had not used CPAP during 2 yr. The patients with sleep apnea in this study had a higher automobile crash rate than all drivers in the state of Colorado (0.07 versus 0. 01 crash per driver per year, p < 0.02). Patients who were treated with nasal CPAP had a lower crash rate while being treated than before treatment (0.07 versus 0 crash per driver per year, p < 0.03). Untreated patients with sleep apnea continued to have a high crash rate (0.07 crash per driver before and after diagnosis). Drivers with sleep apnea were reluctant to report their automobile crashes, for the drivers in this study reported only one-third of the crashes in which they were involved. This is the first study to confirm with traffic records that patients with sleep apnea have fewer automobile crashes while being treated with nasal CPAP.