The association between crowding and aggressive behavior among psychiatric inpatients was investigated. Aggressive incidents were documented on two closed psychiatric wards between February 1 and December 15, 1996. A modest correlation between number of patients on the ward and number of aggressive incidents per patient was found. Enlargement of the physical space by the addition of a courtyard to one of the wards midway through the study did not lead to a significant decline in incidents. Possibly, a lack of psychological space-having no privacy or not being able to get sufficient rest-may be more important in triggering aggression than a lack of physical space.