The authors analyzed the biological samples available in suspected DWI cases between 2000 and 2007. The result of the on-the-spot clinical test is not informative, especially at the simultaneous presence of alcohol. The average age (males 26.92 years, females 27.6 years) of the apprehended drivers is higher than in case of illicit drug users, and male dominance (1652 males--94% and 88 females--6%) is also more expressed. This male-female ratio corresponds to the epidemiological figures of drunken-driving, which may indicate identical driving attitude. Licit or illicit drugs in the urine could be detected in 1293 people out of 1740 (74.31%) whereas in 319 (18.33%) cases there was some substance present in the blood. The occurrence of multiple drug use was high. The use of THC, amphetamine and methamphetamine derivates are the most frequent. An increase in cocaine consumption has been observed for the last few years. The joint use of alcohol and drugs has increased, but the observed blood-alcohol concentrations are much lower than in simple drunken-driving cases. The evaluation of clinical symptoms on its own is not enough to evaluate the effects of drugs, especially when the effect of alcohol is to be considered at the same time.