This paper compares the methods used in Sevilla, Spain with those used in Strasbourg, France for analyzing cannabinoids (delta 9-tetrahydrocannabinol [THC] and 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid [THCCOOH]) in human hair. The Sevilla procedure involved the following steps: After washing the hair samples with methylene chloride at 37 degrees C for 15 min, hydrolysis was performed with 11.8N KOH (in the presence of THC-d3 and THCCOOH-d3) for 10 min at laboratory temperature, followed by the addition of maleic acid, extraction with n-hexane-ethyl acetate, and derivatization with heptafluorobutyric anhydride and hexafluoroisopropanol. Concentrations within the Spanish population ranged from 0.06 to 7.63 ng/mg and from 0.05 to 3.87 ng/mg for THC and THCCOOH, respectively. The Strasbourg procedure involved the following steps: Samples were decontaminated twice with methylene chloride, then pulverized and dissolved in 1N NaOH in the presence of THC-d3 and THCCOOH-d3. After cooling, acetic acid was added, and the samples were extracted with n-hexane-ethyl acetate, followed by a cleanup step with NaOH and HCl. Derivatization with pentafluoropropionic anhydride-pentafluoro-propanol was followed by gas chromatographic-mass spectrometric analysis using negative chemical ionization. Concentrations within the French population ranged from 0.10 to 2.17 ng/mg and from 0.05 to 0.39 ng/mg for THC and THCCOOH, respectively. In order to validate both analytical methods, 10 real hair samples from known drug abusers were analyzed by both procedures. The differences in the results ranged from 2 to 43% (mean, 25.7%) for THC and from 0 to 73.7% (mean, 36.8%) for THCCOOH. After comparing the cannabis results in hair in Spain (70 cases) and France (98 cases), we found quantitative as well as qualitative differences in cannabis consumption in the two populations.