This article describes a study into the occurrence of aggressive behaviour of patients at an acute admission ward of Psychiatric Center Welterhof (Heerlen, the Netherlands). To monitor aggression, the Staff Observation Aggression Scale (SOAS) of Palmstierna and Wistedt (1987) was used. The investigation lasted for 24 weeks during which 164 aggressive incidents were registered. Aggressive behaviour was especially likely to occur in the afternoon and frequently took place in the dayrooms of the ward, in or near the staff office, and before the closed door of the ward. Patients involved in the incidents were younger and more often involuntarily admitted than non-aggressive control patients. It appeared that sex and diagnosis were not related to aggressive behaviour. In practice, the SOAS proved to be a reliable and suitable instrument for measuring aggressive behaviour. This is important because it provides clinicians with an index for measuring effects of interventions aimed at reducing aggressive behaviour.