Caffeine concentrations were measured in the urines of 11 361 athletes tested for doping control in the Ghent doping control laboratory during the period 1993 - 2002. Determination of these concentrations was done using an alkaline extraction with a mixture of dichloromethane and methanol (9:1; v/v) followed by high performance liquid chromatography and ultraviolet detection (HPLC-UV). The method was validated according to ISO 17 025 standards (International Organisation for Standardisation). Quantification was done by using a calibration curve in the range from 0 to 20 microg/ml. The limit of quantification (LOQ) was 0.10 microg/ml. Most caffeine concentrations were far below 12 microg/ml. Because the results were not normally distributed, transformation of the data was done to evaluate the difference in detected concentrations in several sports. This resulted in an overall average concentration of 1.22 microg/ml +/- 2.45 microg/ml. Comparison of those sports with more than 200 samples being analysed demonstrated that caffeine concentrations in urine samples from bodybuilders are significantly higher in comparison to urines taken in the other sports. Also, a significant difference between caffeine concentrations found in cycling and concentrations found in other sports, including athletics and some ball sports, was observed.