The study was conducted over a 12-month period from late 1992 to late 1993. A total of 109 pretrial and 126 sentenced prisoners were randomly selected from a resident population of male inmates in Ireland's largest prison (Mountjoy). Experienced psychiatrists conducted unstructured and semistructured clinical interviews with the inmates, and diagnoses were made according to criteria in the American Diagnostic and Statistical Manual (DSM- III-R). Rates of major mental illness, mainly functional psychosis, were higher than would be expected for the general population. The prevalence of alcohol abuse and other substance abuse, however, occasioned the greatest concern about unmet need. In the absence of longitudinal data, further comment cannot be made about impressions of increase in psychiatric disorder in Irish prisons in the 1980's and 1990's, but there is a major problem with levels of mental disorder in the study population. Health services are apparently not meeting the need. Innovations in mental health legislation and practical approaches to mentally ill offenders are required. 1 table and 15 references