Background : In a companion paper, we established high levels of psychiatric morbidity in prisoners (Bebbington et al. Soc Psychiatry Psychiatr Epidemiol, 2016). In the current report, we evaluate how this morbidity translates into specific needs for treatment and the consequent implications for services. Mental health treatment needs and the extent to which they had been met were assessed in a representative sample of prisoners in a male and a female prison in London (Pentonville and Holloway). ; Methods : Prisoners were sampled at random in a sequential procedure based on the Local Inmate Data System. We targeted equal numbers of male remand, male sentenced, female remand, and female sentenced prisoners. Following structured assessment of psychosis, common mental disorders, PTSD, personality disorders and disorders of abuse, we used the MRC Needs for Care Assessment (NFCAS) to establish whether potential needs for care in ten areas of mental health functioning were met, unmet, or incapable of being met by services. ; Results : Data on treatment experience were provided by 360 inmates. Eighty percent of females and 70% of males had at least one need for treatment. Over half (53.7%) of the needs of female prisoners were met, but only one third (36.5%) in males. Needs for medication were unmet in 32% of cases, while those for psychological treatment were unmet in 51%. ; Conclusions : Unmet needs for mental health treatment and care were common in the two prisons. This has adverse consequences both for individual prisoners and for the effective functioning of the criminal justice system.