Over recent years attention has been directed towards providing appropriate facilities for the mentally abnormal offender. In the past such individuals were probably absorbed into the hospital system. However, the open door policy of the 1960s and 1970s, together with hospital closures and reduced bed numbers, has meant that this invisible absorption has been unable to continue. It has been suggested that many of these mentally abnormal offenders are inappropriately placed within the penal system, thus contributing to prison overcrowding. The Butler and Glancy Reports recommended Regional Secure Units as a possible solution. At the beginning of 1987, 14 of these had opened. The tendency has been for Regional Secure Units to deal mainly with the mentally ill offenders and the needs of the mentally impaired have been largely overlooked. The literature on the subject of this latter group has been sparse. Craft described an isolated unit in Wales, which dealt with mentally abnormal offenders, and there are hospitals which provide secure facilities for the mentally handicapped. The purpose of this study is to describe the characteristics of the patients admitted to an open forensic unit in order to assess the suitability of the facilities and the outcome of the treatment provided.