This study determined the prevalence and features of assaults on staff, compared them with other aggressive incidents by psychiatric in-patients, and studied their relationship with the ward atmosphere. There were 181 physical assaults among 279 staff in two months, i.e. 389 assaults per 100 staff per year. A few patients were responsible for the majority of the assaults. Most assaults were triggered off by staff-patient interaction. About one-third of the staff were significantly psychologically shaken by the incidents. Patients were more likely to be provoked and used more severe means of aggression against staff than against other targets of aggression. There were no significant differences between the characteristics of patients who assaulted staff and those who had other targets of aggression.