To validate information on cannabis use, 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic (THC-COOH) was investigated in human hair. The identification of THC-COOH in hair would document cannabis use more effectively than the detection of the parent drug which might have come from environmental exposure in a smoky atmosphere. Samples (100 mg) were decontaminated with methylene chloride and destroyed by incubation in 1 mL of 1 N sodium hydroxide for 30 min at 95°C in presence of 10 ng of THC-COOH-d₃. After cooling, samples were extracted by n-hexane/ethyl acetate after acidification with acetic acid. After derivatization by PFPA-PFP-OH of the dry extract, drugs were separated on a HP1 capillary column, and detected by mass spectrometry (m/z 602) using negative chemical ionization with methane as reagent gas. Among 30 samples obtained from subjects deceased from fatal heroin overdose, 17 tested positive for THC-COOH, in the range 0.02–0.39 ng/mg, with an average of 0.12 ng/mg.