In Psychiatric Center Welterhof, an aggression-intervention study was carried outbased upon insights from previous research (Nijman et al. 1995). The aim was to reduce the number and severity of the aggressive incidents in a closed acute admissions ward by means of a number of protocolled interventions. Two other closed wardsserved as control wards. The aggressive behavior was registered on a Dutch version of the Staff Observation Aggression Scale (SOAS). It was found that both the total number and th e number of physical aggressive incidents in the ward under study were not significantly decreased compared to the control wards. The severity of the aggressive incidents in the ward under study was found to be less than in the control wards after the implementation of the interventions. This difference was borderline significant. Furthermore, the overall frequency of aggressive incidents in the three wards wasreduced. The question arises whether this reduction occurred under the influence of the SOAS registration.