The coils of operating a psychiatric intensive care unit (PICU) were recorded for a six-month period. There were 110 admissions and 99 discharges. Mean length of stay was 12.3 days. Total costs were £346,516 over the study period, £283.56 per patient-day. Fixed costs comprised 93% of the total, inducing nursing staff ($169,447), overheads ($77,017), medical staff ($48,819), hotel costs (£24,160) and miscellaneous (£1,750). Variable costs included special nursing, (£19,405), treatment of major self-harm (£3,024), drugs (£1,707) and staff time to manage aggressive incidents (£1,188). Reduction of the incidence of aggression could result in valuable cost savings.