Drivers suffering from sleepiness at the wheel run the risk of being involved in car accidents. To evaluate whether objective data of driving performance can be assessed in patients with excessive tendency of falling asleep at the wheel, two test versions of a computerized driving program were created to fit an advanced driving simulator. For the evaluation 15 male drivers with habitual sleep spells whilst driving were selected among patients with the clinical features of the sleep apnoea syndrome. The brake reaction time and the deviations from straight road-line were significantly increased when compared to the performance of 10 matched controls. Irrespective of test version, the driving simulator with the program used was found to be a sensitive method to ascertain driving vigilance impairment in quantitative terms. It would also be a valuable method to evaluate the efficiency of treatment in selected patients.