Impaired simulated driving performance has been demonstrated in obstructive sleep apnoea-hypopnoea syndrome (OSAHS) patients. Although continuous positive airway pressure (CPAP) generally improves simulated driving performance, the effects of oral-appliance (OA) therapy are unknown. The aims of this study were to determine to what extent OSAHS patients have more difficulty with a monotonous simulated driving test when compared with control subjects and to compare the effects of OA with CPAP therapy. Simulated driving performance was evaluated in 20 OSAHS patients and 16 control subjects during a 25-min driving test. After randomization, ten patients started OA and CPAP therapy, respectively. After 2 to 3 months of treatment, patients repeated the driving test. At baseline, the total number of lapses of attention during driving was significantly higher in OSAHS patients as compared with control subjects. As a result of treatment, the total number of lapses of attention was significantly decreased in both the OA and CPAP group. When comparing driving performance between the OA and CPAP group, no significant differences were noted. OSAHS patients perform worse on a simulated driving test when compared with control subjects. When evaluating the effects of treatment, adequate OSAHS management with either OA or CPAP therapy usually resulted in substantial improvements of simulated driving. Conclusions beyond both treatments improving simulated driving performance are, however, not justified by the data in the present study.