In recent years, the interest in the use of oral fluid as a biological matrix has increased significantly, particularly for detecting driving under the influence of drugs (DUID). In this study, the relationship between the oral fluid and the blood concentrations of drugs of abuse in drivers suspected of DUID is discussed. Blood and oral fluid samples were collected from drivers suspected of DUID or stopped during random controls by the police in Belgium, Germany, Finland, and Norway for the ROSITA-2 project. The blood samples were analyzed by gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS), sometimes preceded by immunoassay screening of blood or urine samples. The oral fluid samples were analyzed by GC-MS or LC-MS(/MS). Scatter plots and trend lines of the blood and oral fluid concentrations and the median, mean, range, and SD of the oral fluid to blood (OF:B) ratios were calculated for amphetamines, benzodiazepines, cocaine, opiates, and ▵9-2 tetrahydrocannabinol. The ratios found in this study were comparable with those that were published previously, but the range was wider. The OF:B ratios of basic drugs such as amphetamines, cocaine, and opiates were >1 [amphetamine: median (range) 13 (0.5-182), methylenedioxyamphetamine: 4 (1-15), methylenedioxymethamphetamine: 6 (0.9-88), methamphetamine: 5 (2-23), cocaine: 22 (4-119), benzoylecgonine: 1 (0.2-11), morphine: 2 (0.8-6), and codeine: 10 (0.8-39)]. The ratios for benzodiazepines were very low, as could be expected as they are highly protein bound and weakly acidic, leading to low oral fluid concentrations [diazepam: 0.02 (0.01-0.15), nordiazepam: 0.04 (0.01-0.23), oxazepam: 0.05 (0.03-0.14), and temazepam: 0.1 (0.06-0.54)]. For tetrahydrocannabinol, an OF:B ratio of 15 was found (range 0.01-569). In this study, the time of last administration, the dose, and the route of administration were unknown. Nevertheless, the data reflect the variability of the OF:B ratios in drivers thought to be under the influence of drugs. The wide range of the ratios, however, does not allow reliable calculation of the blood concentrations from oral fluid concentrations.