Background : Aggressive behaviour (i.e. verbal, physical towards objects, self, or others) is highly prevalent in long-term psychiatric inpatient care. We aimed to estimate the overall incidence of aggression, the time staff took to handle aggression incidents, and the weighted average financial costs thereof. Methods : A random sampling procedure was conducted at three long-term psychiatric inpatient care facilities, covering all weekdays and work shifts (day, evening, and night). Psychiatric nurses were asked to recall all incidents of their shift. For the time spent on each type of incident, members of staff were monitored in real-time for a period of one week. Estimated costs were calculated by the time spent multiplied by hourly wages in addition to any material-related costs. Results : The incidence rates were 90 incidents per patient year: 63 for verbal aggression, 8 for physical aggression towards objects, 7 for self-harm, and 12 for physical aggression towards others. The average time spent per incident was 125 minutes but differed for each type of incident. Almost 80% of this time was consumed by nursing staff. The average cost per aggression incident was €78; extrapolated per patient year, the total costs were approximately €7000. Conclusions : The current study found a high rate of aggression incidents in closed long-stay psychiatric wards, particularly when accounting for verbal aggression. Reports of aggression on these types of wards are scarce. Nevertheless, aggression seems to have a severe impact on invested time and related costs, which suggests a need for aggression-prevention and de-escalating programs.