The results of treating psychopaths and other mentally abnormal offenders at Balderton Hospital, Nottinghamshire, and in North Wales (Tables I, II and III), have shown that there is a small intractable group who react explosively when in the company of less disturbed patients, and who are best treated in a long-term unit specially designed for their needs. The small group surveyed here are mentally abnormal offenders of dull-normal or average ability whose aggression or inadequacy, sometimes combined with schizophrenia or resulting from brain damage, makes them prone to repeated convictions for violent acts, arson, sexual offences or drunkenness.