One of the two air distribution systems serving the office rooms of a 25-year-old large office building was balanced in autumn 2002. The supply and extract air flow rates of 173 office rooms were measured before and after balancing. Before balancing, the air flow rates varied considerably from room to room and many of the office rooms were over pressurised. The average extract air flow rate in the whole of the building was 20.7 L/s.person and the standard deviation 11.0 L/s.person. The aim of the balancing process was to meet the design values dating from 1978. The original air distribution plan was very disparate, presumably taking into consideration both room size and number of occupants in the room. Balancing achieved the design air flow rates remarkably well but the person-specific air flow values still varied considerably between the office rooms. After balancing, the average extract air flow rate was 29.3 L/s.person and the standard deviation 13.6 L/s.person. The paper discusses whether design air flow values should be followed when the occupancy within the building has changed and whether energy saving aspects should be taken into account. Balancing is recommended after duct cleaning or major building renovations to accommodate for changes that have taken place in the air distribution system.