When adults with intellectual disabilities (ID) require a psychiatric admission, general adult mental health units are often used. Specialist units have emerged recently as a care option but there is only limited evidence of their effectiveness. Thus this study aims to describe and evaluate the effectiveness of a specialist inpatient unit and report on the utilization of generic and specialist inpatient services. All patients admitted to a specialist ID psychiatric unit were evaluated on admission and immediately after discharge on a number of outcome measures. In addition, they were compared with those admitted to general adult mental health units covering the same catchment area. Significant improvements were demonstrated within the specialist unit cohort on measures including psychopathology, global level of functioning, behavioural impairment and severity of mental illness. The specialist unit patients had a longer length of inpatient stay but were less likely to be discharged to out-of-area residential placement. Specialist units are an effective care option for this group of people.