Objective: Aggressive behaviour 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 long-term psychiatric inpatient care facilities. Nurses were asked to recall all incidents (i.e., verbal, physical towards objects, self, or others) of their shift. For the time spent on each type of incident, staff were monitored in real-time. Estimated costs were calculated by the time spent multiplied by hourly wages in addition to material-related costs. Results: Incidence rates were 90 incidents per patient year. The average time spent per incident was 125 min but differed for each type of incident. Almost 80% of this time was consumed by nursing staff. The average cost per aggression incident was e78; extrapolated per patient year, the total costs were approximately e7000. Conclusions: The current study found a high rate of aggression incidents in closed long-stay psychiatric wards. 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. KEY POINTS Aggression incidents are highly prevalent and are accompanied by high costs. The effect of aggression incidents on the workload for staff members is high, especially for nursing staff. Studies across countries on the incidence and the costs of aggression among psychiatric inpatients are needed to help model the effects of (new) strategies for aggression reduction.