The excretion of marijuana metabolites occurs over an extended period of time, yet few studies have been designed for accurate estimation of excretion half-lives. The authors monitored excretion of the primary urinary metabolite of marijuana, 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THCCOOH), by gas chromatography-mass spectrometry in a controlled clinical study of marijuana smoking that included measurement of the drug in each urine void collected during the 3-week study. Terminal excretion half-lives of THCCOOH were determined in six healthy male subjects with histories of marijuana smoking; the study was conducted on the clinical research unit of a major medical institution. Subjects smoked a single marijuana cigarette (placebo, 1.75% or 3.55% THC) each week. Urine specimens (N = 953) were analyzed under blind conditions for THCCOOH by gas chromatography-mass spectrometry. Mean ± SEM half-lives calculated by the amount remaining to be excreted method after the low and high doses were 31.5 ± 1.0 hours (range, 28.4 to 35.3 hours) and 28.6 ± 1.5 hours (range, 24.9 to 34.5 hours), respectively, when a 7-day monitoring period was used. The amounts of THCCOOH excreted over a 7-day period were 93.9 ± 24.5 µg (range, 34.6 to 171.6 µg) and 197.4 ± 33.6 µg after the low- and high-dose sessions. Longer half-lives, 44.3 to 59.9 hours, were obtained with a 14-day sample collection. This study documents the prolonged excretion of THCCOOH in urine and emphasizes the importance of study design in the precise estimation of terminal excretion half-lives. A sensitive analytical method and a prolonged specimen collection period are important study considerations in the monitoring of marijuana excretion.