OBJECTIVE--To describe the prevalence of psychiatric disorder and the treatment needs of sentenced prisoners in England and Wales. DESIGN--Population survey based on a 5% sample of men serving prison sentences. SETTING--Sixteen prisons for adult males and nine institutions for male young offenders representative of all prisons in prison type, security levels, and length of sentences. SUBJECTS--406 young offenders and 1478 adult men, 404 and 1365 of whom agreed to be interviewed. MAIN OUTCOME MEASURES--History of psychiatric disorder, clinical diagnosis of psychiatrist, and required treatment. RESULTS--652 (37%) men had psychiatric disorders diagnosed, of whom 15 (0.8%) had organic disorders, 34 (2%) psychosis, 105 (6%) neurosis, 177 (10%) personality disorder, and 407 (23%) substance misuse. 52 (3%) were judged to require transfer to hospital for psychiatric treatment, 96 (5%) required treatment in a therapeutic community setting, and a further 176 (10%) required further psychiatric assessment or treatment within prison. CONCLUSIONS--By extrapolation the sentenced prison population includes over 700 men with psychosis, and around 1100 who would warrant transfer to hospital for psychiatric treatment. Provision of secure treatment facilities, particularly long term medium secure units, needs to be improved. Services for people with personality, sexual, and substance misuse disorders should be developed in both prisons and the health service.