OBJECTIVES: To determine prevalence of mental disorder among male unconvicted prisoners and to assess the treatment needs of this population. DESIGN: Semi-structured interview and case note review of randomly selected cross section of male remand population. Non-attenders were replaced by the next name on prison roll. SETTING: Three young offenders' institutions and 13 adult men's prisons. SUBJECTS: 750 prisoners, representing 9.4% cross sectional sample of male unconvicted population. MAIN OUTCOME MEASURES: Prevalence of ICD-10 diagnoses of mental disorder, and associated treatment needs. RESULTS: Psychiatric disorder was diagnosed in 469 (63%) inmates. The main diagnoses were: substance misuse, 285 (38%); neurotic illness, 192 (26%); personality disorder, 84 (11%); psychosis, 36 (5%); other and uncertain, 36 (0.5%). Subjects could have more than one diagnosis. The average refusal rate was 18%. In total 414 inmates (55%) were judged to have an immediate treatment need: transfer to an NHS bed, 64 (9%); treatment by prison health care services, 131 (17%); motivational interviewing for substance misuse, 115 (15%); and therapeutic community placement, 104 (14%). CONCLUSIONS: Mental disorder was common among male unconvicted prisoners. Psychosis was present at four or five times the level found in the general population. Extrapolation of our results suggests that remand population as a whole probably contains about 680 men who need transfer to hospital for psychiatric treatment, including about 380 prisoners with serious mental illness.