Whole-building pressurisation tests can quantify the air-leakiness of a building's external envelope. The resulting information can be used in assessing the quality of the building fabric. At present there is little information regarding the leakage characteristics of large, non-domestic UK buldings. As a step towards providing more information, the Building Research Establishment (BRE) has developed and constructed a multifan pressurisation system known as BREFAN to pressurise large buildings like offices and hangars. This paper presents results from field measurements in five large and naturally ventilated buildings. Using BREFAN, measurements in a medium-sized building specifically designed and constructed as a low-energy office (LEO), showed a reduction of 9% (at a pressure difference of 25 Pa between inside and outside) in its envelope leakiness after new and improved windows were installed as part of a programme of modifications. Using QZ5/S (whole building leakage rate at a 25 Pa pressure differential per unit permeable surface area) as an index of the leakiness of the building envelope, these results, when compared with measurements made previously in a conventional UK office, showed that the LEO building was twice as tight as the more conventional building. Comparison with measurements made in North American showed the LEO building was as tight as buildings found there. BREFAN measurements in a second building, a conventional hangar built in the early 1960's, showed it to be very leaky. At 25 Pa pressure difference, the opening of a roof vent increased the leakiness by 17% and unsealing a large folding door increased it by 8%. The envelope leakiness of the hangar was compared with measurements in three other large single-cell factory buildings using a different system of fans to pressurise the buildings. One of these, built 35 years ago, was as leaky as the hangar building. The other two buildings, built within the last decade under current UK Building Regulations, were shown to be twice as tight.