Capture-recapture methodologies have been used to estimate the size of the hidden population of active offenders on the basis of the observed properties of the truncated distribution of arrested offenders. We use this approach to estimate the odds of arrest of marijuana, cocaine, crack, and heroin dealers and users in one Canadian province (Quebec). Findings indicate that risks of being arrested are much higher for sellers than for consumers and that this gap widens for the more harmful drugs. Findings also show, however, that vulnerability to arrest was significantly higher for marijuana users than for others users and that dealers in the smaller but more harmful drug markets (crack and heroin) manage to experience lower aggregate risks of arrest than cocaine or marijuana dealers.