To date there have been no studies estimating the hidden prevelance of opiate users in Dublin. In this study a multiscore enumeration followed by the application of the capture-recapture method with log-linear modelling including age and gender stratification to remove heterogeneity was implemented to provide an estimate of the unknown size of the opiate-using population. Two medical and one legal data sources were used. It was found that the ratio of known to unknown opiate users 1:1.15 with a total of 13,460 (95% Cl: 12,037-15,306) users estimated in Dublin in 1996. The authors conclude that the findings of this study have important ramifications for service delivery.