Although previous studies have shown that patients with obstructive sleep apnea have a higher automobile crash rate than normal subjects, objective measurements of driving performance in patients with sleep apnea have not been reported. Therefore, we compared the driving performance of subjects with untreated, severe sleep apnea to that of control subjects on two driving simulators. Using a simulator with road films, six subjects with untreated, severe apnea performed worse than did a control group of seven normal subjects on both highway and city/rural driving (p less than 0.05). Using a personal computer program simulating a monotonous highway drive, 12 subjects with untreated sleep apnea performed worse than 12 control subjects. The patients with apnea hit a greater number of road obstacles during their 30-minute simulated drive than did the control subjects (44 +/- 52 in patients with apnea versus 9 +/- 7 in control subjects, p less than 0.05). Six patients with apnea hit fewer road obstacles after treatment with nasal continuous positive airway pressure (CPAP) than before treatment (29 +/- 19 before CPAP versus 13 +/- 8 after CPAP, p less than 0.05). We conclude that: (1) driving simulator performance of untreated subjects with severe obstructive sleep apnea is worse than that of control subjects; (2) driving simulator performance of subjects treated with nasal CPAP improves.