A gas chromatographic/mass spectrometric electron impact method is presented for the detection and quantification of 11-nor-9-carboxy-delta 9-tetrahydrocannabinol (THC-COOH) in urine, for use in the confirmation of presumptive results obtained by other techniques. Four extraction procedures, two solid-liquid and two liquid-liquid, have been compared. A comparison of two trimethylsilylating methods demonstrates that the best results are obtained by the use of a mixture containing N-methyl-N-trimethylsilyl-trifluoroacetamide, trimethyliodosilane and dithioerithritol (100:0.2:1) v/v/w. The use of ketoprofen as a new internal standard for the quantification of THC-COOH has proved to be very effective. Both spiked samples and samples from cannabis users have been successfully analysed. It has also been demonstrated that the presence of other drugs of abuse in urine samples do not interfere with cannabis quantification by the method reported here.