Ventilation in high-rise residential buildings in Hong Kong was studied. Overnight carbon dioxide levels and outdoor ventilation rates were studied in bedrooms fitted with air conditioners. The ventilation rate did not meet the requirements specified in the American Society of Heating, Refrigeration and Air-Conditioning Engineers ( ASHRAE ) standard 62-2001, even for rooms with only one occupant. Laboratory studies were undertaken using a typical room which was fitted with a window-type ( WRAC ) or a split-type air conditioner. The ventilation damper currently fitted to WRAC units did not effectively control the outdoor ventilation rate. Its use has the added disadvantage of wasting energy, as the air which it exhausts has just been cooled by passage over the cooling coil. An improved design is proposed, which eliminates this shortcoming. Appropriate bedroom ventilation rates are discussed, and a rate of 3.0 litre/s per person is proposed. 23 refs.