In spring 2008, an influenza A subtype H3N2 outbreak occurred in a long stay psycho-geriatric ward and two wards in the intellectual disability services (IDS), part of a large psychiatric hospital. The attack rate in the index ward was 90% (18/20) for patients and 35% (7/20) for staff. It was 14% (1/7) and 17% (2/12) in the affected IDS wards for patients and 0% (0/20) and 4% (1/25) for staff. Many of the laboratory-confirmed cases did not have a fever >38 °C, a typical sign of influenza. Control measures included oseltamivir treatment for cases and prophylaxis for contacts, standard and droplet infection control precautions, active surveillance for early detection and isolation of potential cases. As a result, the outbreak did not spread throughout the hospital. Although the staff vaccination rate (10%) prior to the outbreak was low, we observed a much lower vaccine effectiveness rate in the patients (11%) than in the staff (100%) in the index ward. Vaccination of residents and staff of such facilities remains the key influenza prevention strategy.