This work presents the results of measurements of the air leakage of 12 large office buildings. Various different construction types have been tested, with 4 of the buildings being naturally ventilated and the remainder air-conditioned. Ideally, air-conditioned buildings should have minimal air infiltration, and naturally ventilated buildings should have air infiltration under occupant control. The results of this research indicate that for relatively large office buildings in the United Kingdom there are openings left in the structure which are on average equivalent to 5 square metres (54 square feet). The average normalized leakage of these buildings was 21.80 M3.hr-1.m-2 at an average envelope test pressure of 50 Pascals. The naturally ventilated buildings tended to be tighter than air-conditioned buildings and pre-1990 buildings tended to be tighter than post-1990 buildings. BSRIA recommends that all new buildings should have a maximum air leakage of 10 m3.hr-1.m-2 and air-conditioned or low-energy buildings should have a maximum air leakage of 5 m3.hr-1.m-2 at a test pressure of 50 Pascals.