A capillary gas chromatography mass spectrometry assay has been developed for the determination of 6-monoacetylmorphine in human urine, which is a confirmatory marker of heroin abuse. It was extracted with chloroform-isopropanol-n-heptane (50:17:33, v/v) from alkalinized samples (pH 9.2), using levallorphan as the internal standard. After derivatization with trifluoroacetic anhydride, the drugs were separated on a 25-m capillary column BP 10 and detected by selected ion monitoring.