This study of violent behaviour among psychiatric in-patients in three hospitals found that many incidents of petty violence occurred and serious violence was rare. Among the offending patients there were more females than males, more young than old and the most commonly represented diagnosis was schizophrenia. Nursing staff were by far the commonest victims. Most of the incidents occurred during the daytime and were perpetrated mainly by patients nursed on acute psychiatric wards. The vast majority of patients were nonviolent.