In Belgium, the driving under the influence of drugs (DUID) procedure consists of three steps: observation of external signs of drug consumption by a police officer; an on-site urine test for amphetamines, cannabinoids, cocaine, and opiates; and blood sampling by a physician for gas chromatography-mass spectrometry analysis. The driver is sanctioned if THC is greater than 2 ng/mL, morphine is greater than 20 ng/mL, or amphetamine, methylenedioxymethamphetamine (MDMA), methylenedioxyethylamphetamine, N-methyl-1-(3.4-methylenedioxyphenyl)-2-butanamine, cocaine, or benzoylecgonine are greater than 50 ng/mL in plasma. We analyzed the results of 450 blood samples taken from May 2000 to February 2005. Cannabis was most often found above the cut-off (73.5% of the cases), followed by MDMA (20.4%), amphetamine (19.8%), benzoylecgonine (17.9%), cocaine (6.9%), and morphine (2.7%). One drug was found in 72.0% of the cases, two drugs in 22.6%, three drugs in 5.2%, and four drugs in 0.25%. In 10.7% of the plasma samples no target drugs were found above the legal cut-off. This percentage was 8.4% when urine was obtained and tested on-site and 21.2% when no urine was obtained (chi2 = 8.574, P = 0.0034). In 64.6% of these samples, a target drug (THC in 74.2%) was found under the legal cut-off. These data indicate that roadside urine testing significantly decreases the number of unnecessary blood analyses in DUID.