This paper provides the first estimates of recorded and hidden prevalence of opiate use among young people in Dublin. It explores the nature of other drugs used and their effect on the decision to leave school early. The author implements the first multisource enumeration of opiate use in young people in Dublin in 1996 and 1997. Hidden prevalence is estimated by the capture–recapture methodology. The nature of other drugs used was examined by surveying early school leavers. A minimum of 1528 young people aged between 10 and 20 years were identified as using opiates through three 1996 data sources. Using three samples it was estimated that 4081 (95% CI of 3586–4692) were using opiates in Dublin in 1996. Using two samples it was estimated that 1141 (95% confidence interval (CI) of 901–1381) of these were problematic users. Using the two-sample method it was estimated that this rose to 1315 problematic users in 1997. It was found that 51.1% of those surveyed had tried using drugs before they had left school and 46.5% noted that this use had affected them while they attended school. The implications for policy of the size of the opiate problem among young people of school age is considerable.