Objective: Until July 2007, the driving under the influence of drugs (DUID) legislation in Denmark was based on impairment, evaluated on the basis of a clinical investigation and toxicological analyses, but in 2007 fixed concentration limits were introduced into the Danish traffic legislation. The objective for this study was to investigate the prevalence of medication and illicit drugs among Danish drivers before and after 2007. Methods: Blood samples from drivers suspected of being under the influence of medication and/or illicit drugs were investigated as requested by the police. The results for a 10-year period before and for one year after the introduction of fixed concentration limits are presented. Results: A total of 2340 blood samples were analyzed for the presence of medications and/or illicit drugs for the period 1997-2006. The average number of cases per year was 234 (213-283), and on average 87 percent of the investigated cases were positive for one or more drugs. For 2008 the number of investigated traffic cases was increased to 1176. Seventy-three percent of the cases from 2008 were positive for one or more drugs. Benzodiazepines, cannabis (THC), amphetamine, heroin/morphine, methadone, cocaine, and ecstasy were the most frequently detected drugs for the period 1997-2006 and also in 2008. The number of these cases in which an ethanol level was detected above 0.5 mg/g (the Danish legal limit) was on average 18 percent (9-26%) for the period 1997-2006 and 19 percent for 2008. The average age of the drivers ranged from 31 to 34 years for the period 1997-2006 and was 31 years for 2008. The percentage of females per year ranged from 3 to 20 percent. Conclusion: The number of traffic cases investigated for substances other than ethanol were consistently low, in the range of 200 to 300 per year during the period from 1997 to 2006, but after the introduction of fixed concentration limits in 2007 a 5-fold increase was seen already in 2008.