OBJECTIVE: This study examined whether staff members' use of interventions for preventing patients' aggression reduced the number of incidents of aggression on a closed acute admissions ward of a psychiatric hospital in the Netherlands. METHODS: The number and severity of incidents of aggression on three closed wards were measured using the Dutch version of the Staff Observation Aggression Scale (SOAS). Measurements were made for three months before and after staff implemented interventions for preventing aggression on one of the wards. Interventions included a protocol for talking to patients who exhibited aggressive behavior, discussing treatment goals with the patient shortly after admission, explaining why the ward's door was locked and the exit rules, providing a schedule of staff meetings to explain staff members' absence from the ward, and clarifying the procedure for making an appointment with the psychiatrists. RESULTS: The frequency of aggressive incidents was reduced on all three wards, with no significant difference between the ward where the interventions were implemented and the two control wards. A marginally significant difference in the severity of aggressive incidents was found between experimental and control wards after the introduction of the preventive measures, with incidents in the experimental ward tending to be less severe. CONCLUSIONS: This study failed to find a robust effect of specific intervention measures on aggressive incidents. However, it did find evidence suggesting that standardized reporting by staff of aggressive incidents on closed psychiatric wards may in itself result in straightforward reduction of violent incidents.