Public health planning continues to be troubled by the uncertainty of the extent of hidden drug use. Methods for estimating the prevalence of opiate use are discussed. These include multisource enumeration, death multiplier, multiple indicator, and capture-recapture methods. The feasibility and data requirements for each of these methods is illustrated for the first time in an Irish context. Estimates presented are the result of years of intensive collaboration between previously unconnected government, health, and legal agencies. Finally, the implications of the methods and their results for the planning and provision of medical and social policy are highlighted and discussed.