During a 25-week period, all incidents of aggressive behavior in a 19-bed psychiatric acute care unit were systematically recorded using the Staff Observation Aggression Scale. Forty-seven of the 163 patients admitted to the unit were aggressive on 119 occasions. Of these incidents, 100 were physical attacks on another person, and 95 were perpetrated by 23 patients. The patients were predominantly aggressive without visible provocation or were provoked by staff's denying a request. An increased number of patients on the ward significantly increased the likelihood of aggressive behavior, especially by patients with schizophrenia or schizophreniform disorder.