This report presents the findings of a survey of drug-using offenders in Mountjoy prison, conducted in July 1986. One in every five offenders in the prison was randomly selected to take part in the survey, yielding a total of 110 men. In all, 95 people agreed to take part in the study, and 29 of these were found to have a history of serious drug abuse. This proportion of drug users represented an almost six-fold increase in the number of serious drug users in the prison since a similar survey carried out by the author five years previously. Opiates were found to be the main drugs of use, usually heroin or dicanol, which were injected in most cases. The mean age of the drug-using prisoners was 25 years, and the average duration of drug use for daily users of opiates was 7.1 years. Only about 20% of the group were sentenced specifically for drug-related offences; most were serving sentences for different forms of theft. Slightly more than one in five of the subjects were found to have been successfully and stably 'drug free' for at least one year prior to their current imprisonment, although this finding did not seem to be strongly related to formal therapy for drug abuse.