A retrospective study of recordings of all violent episodes among in-patients requiring restraint was undertaken over a two-year period during which the nature of admissions to the unit changed to an increase in mentally abnormal offenders (those admitted through the criminal justice system and detained under criminal sections of the Mental Health Act). It was found that those detained under criminal sections were less likely to be aggressive, were of relatively older age and less likely to repeat the violence than their mentally ill counterparts detained under civil sections. In addition to the past history of violence, verbally abusive behaviour was the best predictor of subsequent physical aggression though only a minority of patients displaying serious physical assault showed verbally abusive behaviour. Deterioration in mental health and staff-patient interaction were seen as major contributory factors to aggression which will also explain a greater degree of aggression directed towards the staff. Further comparative analysis of aggressive behaviour of mentally ill patients detained under civil sections and mentally abnormal offenders is warranted.