The paper compares the prevalence of psychiatric morbidity amongst sentenced prisoners and in the general population. Major psychosis was no more common in the majority of studies of criminal populations. Although prisoners have a higher level of neurotic symptomatology, this was mainly found to be secondary to imprisonment itself. Long term imprisonment was not found to be a precipitant of severe psychiatric morbidity or intellectual deterioration, and prisoners adopt elaborate coping mechanisms which may themselves be protective. However, there is a higher prevalence of mentally handicapped and epileptic prisoners, and doctors in the Prison Medical Service have to cope with frequent, serious behavioural problems. Prisons appear to be a particularly important area for future psychiatric research.