BACKGROUND Because millions of people are self-prescribing melatonin for various indications, the safety aspects of this substance have become very important. The aim of our study was to determine whether or not melatonin impairs driving-related performance. METHODS Twenty healthy men and women aged 21-57 years volunteered for this randomized, placebo-controlled, double-blind, crossover study. The crossover arms were separated by an interval of at least 4 weeks. On each testing day, melatonin 5 mg or placebo was taken at 1630 h; 60 minutes later a test series was performed, consisting of a medical examination, body sway measurement, and a standardized driving computer test battery to assess attention, reaction time, power of concentration, and sensomotor coordination. Subjective sleepiness was measured on three occasions during the test session using the Stanford Sleepiness Scale questionnaire. RESULTS Just one of the 16 main variables of the driving computer test battery, the selective attention tested by signal-detection, was significantly affected by melatonin (p < .05). However, even those values were still within the normal range. Subjective sleepiness was increased by melatonin, although the result was significant only after the prolonged concentration task (p < .05). Neither the clinical examination nor the body sway test showed signs of any drug influence. CONCLUSIONS The overall result of the computer test battery showed no objective adverse impact of melatonin on driving performance. However, due to the increased subjective sleepiness after administration of this hormone, caution should be exercised when driving under the influence of melatonin.