Abstract. Patients with obstructive sleep apnea syndrome (OSAS) have an accident rate between two and seven times higher than normals. Investigations on accident frequency are based on case history, insurancy reports, and driving simulator investigations. The present controlled study was planned to test whether an increased accident risk could be demonstrated in patients with OSAS before and on CPAP (continuous positive airway pressure)-therapy using the driving simulator C.A.R. Driving simulator performance was investigated in 31 patients with polysomnographically confirmed OSAS (apnea-hypopnea-index 24.8 +/- 21.5/h) before, 2 and 42 days after initiation of CPAP and was compared to 10 healthy controls in whom OSAS was excluded by polysomnography. Driving simulator performance was significantly worse in OSAS as compared to normals especially in terms of accident frequency (OSAS: 2.7 +/- 2.0, controls: 1.3 +/- 1.5, p < 0.05) and concentration faults (OSAS: 12.4 +/- 5.1, controls: 7.1 +/- 3.2, p < 0.01). On CPAP accident frequency (OSAS before therapy: 12.4 +/- 5.1, 2 days CPAP: 1.5 +/- 1.4, p < 0.01; 42 days CPAP: 0.9 +/- 1.3, p < 0.001) and frequency of concentration faults (OSAS before therapy: 12.4 +/- 5.1, 2 days CPAP: 6.5 +/- 3.9, p < 0.001; 42 days CPAP: 4.9 +/- 3.3, p < 0.001) could be lowered significantly both in the short and medium term of therapy. The driving simulator C.A.R. is an adequate tool for the evaluation of an increased accident risk in OSAS-patients and demonstrates the efficiency of CPAP-therapy.