Among the many difficulties with estimating the size of the cannabis industry is that suitable methodologies for estimating large-scale outdoor illegal drug production in developing countries cannot be used to estimate indoor production in industrialized countries. This article proposes a new approach that overcomes some of these difficulties. The case study is a mature cannabis cultivation industry,located in the province Quebec, Canada. Starting from capturerecapture estimates of the prevalence of growers, the approach combines police and fieldwork data sources on the dynamics of the cultivation industry to correct for typical errors in the assumed productivity rates ofdifferent kinds of cultivation sites. Using three different approaches to productivity (ounces-per-plant, yield-perlamp, yield-per-watt) it was estimated that Quebec cannabis production was approximately 300 tons in 2002; 11% was seized by the police, 33% was consumed within the province, and 56% was potentially exported to the U.S. and to other Canadian provinces.