Aggressive behaviour by psychiatric inpatients was investigated on the same ward during two separate 6-month periods before and after a 50% decrease in number of beds but without changes in catchment area. Character of the patient group, patient turnover and medical practices as well as total number of staff on duty on the ward was virtually the same during the two periods. It is therefore assumed that differences in aggressive behaviour are mainly explained by effects due to the decreased number of beds per se. It was found that the 50% reduction did not affect the overall aggression frequency. However, inter-patient violence increased while the number of more severe aggressive incidents towards staff members decreased.