The primary aim of this study was to compare task performance in a laboratory test and real driving performance of outpatients receiving diazepam medication with those of control subjects. Plasma and saliva samples were taken to investigate a level-response relationship. Real driving performance was measured by trained observers. The design of the laboratory test was based on a vigilance task (high attention) directly followed by a simple eye-hand coordination tasks (low attention). Twenty-two males participated in the study. Diazepam was given orally by prescription, mostly as a maintenance dose of 5 mg three times a day. Patients receiving diazepam showed impaired performance in the driving test and the low-attention task. Furthermore, the results indicate no relationship between plasma or saliva levels of diazepam and/or its metabolite N-desmethyldiazepam and real driving performance and/or laboratory task performance.