The purpose of this study was to provide estimates of the number of 'problem heroin users' in Northern Ireland during a 12-month period, 1 November 2000 to 31 October 2001. The study used three methodological techniques to generate estimates of the number of problem heroin users in Northern Ireland. Each technique utilised the number of known users to estimate the hidden population of problem heroin users. These techniques included capture recapture methodology, a mortality multiplier and an Addicts Index multiplier.