The frequency of motor vehicle and working accidents was analyzed by means of a strictly anonymous questionnaire in 156 patients with sleep apnea syndrome (SAS) and in 160 age-gender matched controls. In the SAS group 12.4% of all drivers had motor vehicle accidents as compared to 2.9% in the control group (p<0.005). The motor vehicle accident rate was 13.0 per million km in patients with more severe SAS (AHI > 34/h, n=78) as compared to 1.1 in patients with milder SAS (AHI 10-34/h, n=78) (p<0.05), and 0.78 in control group (p<0.005), respectively. The accident rates in both patients and the control group were also greater than the rate of 0.02 "accidents due to sleepiness" per one million km in the Swiss driving population as reported by official statistics. During treatment with nasal continuous airway pressure (nCPAP) in 85 SAS patients, the motor vehicle accident rate dropped from 10.6 to 2.7 per million km (p<0.05). We conclude that patients with moderate to severe SAS have an up to fifteen-fold risk increase of motor vehicle accidents that constitutes a serious and often underestimated hazard on the roads, which can be reduced by adequate treatment.