Mentally ill prisoners have consistently been highlighted as a group with complex needs. However, it is not clear what these needs are, how effective prison health services are in meeting the needs of this vulnerable group, and whether there would be any benefit to transferring them to psychiatric facilities. This study compared the characteristics and needs of mentally ill patients in prison healthcare centres (HCC prisoners) with patients in forensic medium secure psychiatric units (MSU inpatients) in the UK. HCC prisoners and MSU inpatients were in fact very similar. Where they did differ, though, was that MSU inpatients were significantly more likely to be of non-white ethnicity, diagnosed with psychotic or substance use disorders, and have recorded histories of drug misuse. HCC prisoners and MSU inpatients reported the same number of needs overall, according to the CANFOR-S (Camberwell Assessment of Need Forensic Short Version), but HCC prisoners reported significantly higher levels of unmet need. The need for transfer to alternative services was common in both groups, with bottlenecks evident at all levels. The high level of unmet need reported in the HCC prison sample is of particular clinical concern and highlights the need for improved assessment and treatment services in this setting.