BACKGROUND: About 3-7% of the adult population receives prescribed hypnotics. The benzodiazepine-like hypnotics, zopiclone and zolpidem, are the most commonly prescribed and may cause traffic-relevant impairment similar to that found for benzodiazepines. We investigated the relationship between blood zopiclone and zolpidem concentrations and driving impairment, as assessed by the clinical test for impairment. We compared these groups of drivers to a group suspected of alcohol-related impairment. METHODS: Blood samples from suspected impaired drivers during 2000-2007, screened for approximately 25 possible impairing drugs with only one single drug detected, were studied in relation to the assessment of impairment. The 79 zopiclone positive drivers, the 43 zolpidem positive drivers, and the 3480 alcohol positive drivers were divided into groups depending on blood drug concentrations. RESULTS/DISCUSSION: The proportion of drivers judged as impaired tended to increase the higher the blood zopiclone concentrations. Such a positive relationship was not found for zolpidem. For alcohol the proportion of impaired drivers was significantly related to blood alcohol concentrations (BACs). There were few drivers with low zopiclone or zolpidem concentrations included, which may have obscured any positive significant relationship. The percentage of impaired drivers among drivers with blood zopiclone concentrations above 130 microg/l roughly corresponded to the percentage of impaired drivers among drivers with BACs higher than 0.1%.