This paper examines the feasibility of using information on treatment admissions to infer the size of a heroin-using population. Several variants of capture-recapture and truncated Poisson models which have been used for this purpose are assessed here, and are generally found to be unsuited to the task at hand. A cohort model is developed which is consistent with the dynamics of drug use and help-seeking behavior, and which would allow estimation of the size of the treatment susceptible or "inbound" population, but its information requirements are difficult to meet. The analysis suggests that capture-recapture and truncated Poisson models should be evaluated in terms of their potential for reformulation and application within the present context, or alternatively, that other methods be developed for this purpose.