This study reports on a 1-yr study of 235 adult male prisoners in the Republic of Ireland to ascertain the impact of changes in patterns of health service for mental disorders on the prison population. Data were collected on incoming prisoner's level of illness or distress to help with the planning of future prison health services. Experienced psychiatrists conducted unstructured and semistructured clinical interviews and made diagnoses using criteria in the Mental Disorders-III-Revised (DSM-III-R). Rates of major mental illness, mainly functional psychosis, were higher than would be expected for the general population, but it was the prevalence of alcohol and substance misuse that gave rise to the greatest concern about unmet need. Innovations in mental health legislation and new practice approaches for conditions of alcoholism and other substance abuse problems are urgently needed.