Differences between patients being treated in two different medium secure units, one for those of normal intelligence and one for those with learning disability, were investigated. Sociodemographic, psychiatric, medical and medicolegal data were recorded for all inpatients in both secure units. Patients in the medium secure unit for those of normal intelligence were older at the time of the study and at conviction for the index offence, more likely to have been admitted from the penal system, and more likely to suffer from a psychotic disorder, particularly schizophrenia or a mood disorder. Their index offence was more likely to be homicide, attempted murder, manslaughter or grievous bodily harm, while that of the patients with learning disability was more likely to be a sexual offence. A need for separate medium secure unit facilities for those of normal intelligence and those with learning disability is supported by differing psychiatric and behavioural requirements.