Patients with clinical features of sleep apnea syndrome (SAS) and self-reported sleep spells at the wheel do poorly in simulated monotonous driving. To evaluate whether drivers with defined symptoms of SAS (heavy snoring, sleep disturbances and daytime sleepiness) compensate in real traffic by careful driving or not, the rate of car accidents over a 5-year period was investigated. A questionnaire was addressed to 140 patients with and 142 controls without symptoms associated to SAS. Seventy-three of the patients had a complete triad of SAS-associated symptoms. Fifty-two percent of these patients reported habitual sleep spells at the wheel, as opposed to less than one percent by the controls. The ratio of drivers being involved in one or more combined-car accident was similar for patients and control drivers, but for single-car accidents the ratio was about 7 times higher for patients with a complete triad of symptoms of SAS compared to controls (p less than 0.001). When corrected for mileage driven, the total number of single-car accidents was almost 12 times higher among patients with sleep spells whilst driving, compared to controls (p less than 0.001). It is concluded that drivers with the clinical features of SAS are at increased risk especially for single-car accidents and that the risk seems to vary with the severity of symptoms.