The paper describes a case-note and interview study of a cross-sectional sample comprising 25% of all women serving a prison sentence in England and Wales. A 5% sample of the male sentenced prison population was used for comparison. Diagnoses were assigned on clinical grounds and an assessment was made of the treatment needs of all 'cases'. The prevalence of psychosis, around 2%, was similar in the two groups but women had higher rates of mental handicap (6% v. 2%), personality disorder (18% v. 10%), neurosis (18% v. 10%) and substance abuse (26% v. 12%). There is a need for closer links between the NHS and prison health services. Women's prisons lack a therapeutic community of the Grendon type, which may be of benefit to a substantial minority of inmates.