An accurate, reproducible, and validated gas chromatography-mass spectrometry (GC-MS) method for the quantitation of 11 -nor-delta9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH), the major metabolite of delta9-tetrahydrocannabinol, in urine is described. Extraction was performed with n-hexane/ethyl acetate. Deuterated THC-COOH was used as the internal standard. The GC-MS analysis was done by selected ion monitoring. No interferences were detected in 20 blank urine samples of different origin. The calibration curve was found to be linear over the range of 10-100 ng/mL. The calculated limits of detection and quantitation were 1.0 ng/mL and 1.7 ng/mL, respectively. Results of positive findings for cannabis use in doping control in Flanders and Portugal in the period of 1997-2000 are commented.