To validate information on cannabis use, we investigated human hair and pubic hair for cannabinoids (THC and THC-COOH) by gas chromatography/mass spectrometry. Samples (100 mg approximately) were decontaminated with methylene chloride, then pulverized and dissolved in 1 ml 1 N NaOH for 10 min at 95 degrees C in the presence of 200 ng of deuterated standards. After cooling, samples were extracted by n-hexane/ethyl acetate after acidification with acetic acid. After derivatization of the dry extract by PFPA/PFP-OH, the drugs were separated on a 30-m capillary column and detected using selected-ion monitoring (m/z 377 and 459 for THC and THC-COOH, respectively). Forty-three hair samples were obtained from fatal heroin overdose cases. Among them, 35% tested positive for cannabinoids. Hair concentrations ranged from 0.26 to 2.17 ng/mg (mean, 0.74 ng/mg) and 0.07 to 0.33 ng/mg (mean, 0.16 ng/mg) of THC and THC-COOH, respectively. As is generally the case for other drugs detected in hair, metabolite concentration was always lower when compared to the parent drug concentration. In public hair, THC concentrations ranged from 0.34 to 3.91 ng/mg (mean, 1.35 ng/mg) and THC-COOH concentrations from 0.07 to 0.83 ng/mg (mean, 0.28 ng/mg). In most cases, the highest cannabinoid concentration was found in pubic hair, suggesting that this sample may be the more suitable for cannabis testing.