To assist in determining ability to drive in patients with obstructive sleep apnea (OSA), we developed a divided attention driving test (DADT) based on the work of Moskowitz and Burns (6). We first examined its ability to detect impaired performance by testing normal subjects both sober and impaired by alcohol (mean blood alcohol level, 95 +/- 25 mg/dl). Subsequently, 21 male patients with OSA (age 49.3 +/- 12.7 [SD] yr; apnea hypopnea index [AHI] 73 +/- 29) and 21 age- and sex-matched control subjects underwent polysomnography followed by daytime sleep latency testing (MSLT). Before each day-time nap, subjects were given the DADT for 20 min. Patients who performed much worse than control subjects in all measures, with the largest difference noted in tracking error (OSA, 228 +/- 145 cm versus control 71 +/- 31 cm, p < 1 x 10(-9)). Half of the patients were worse than any control subject, with some showing performance worse than control subjects impaired by alcohol. However, MSLT and AHI explained less than 25% of the variance in tracking error, making individual prediction problematic. We concluded that in laboratory driving performance skills are markedly impaired in over half our group with sleep apnea. Further testing and comparing on-road performance should aid in predicting ability to drive.