In countries such as the United Kingdom that possess a temperate climate, the majority of buildings are not air-conditioned but depend upon natural ventilation and relatively simple heating systems. Openable windows are used to reduce the incidence of summertime overheating. Studies on housing have indicated, however, that window opening also occurs extensively through the heating season resulting in substantial heat loss and consequent energy consumption. This paper describes the results of an initial investigation to determine whether the same phenomenon occurs in office buildings. Observations were made of the use of windows in five office buildings over a three-month period, covering a range of weather conditions. The proportion of windows open was found to be closely correlated with external air temperature and to be affected, also, by solar gain and wind speed. A more detailed analysis suggested two modes of window use, one related to the need to maintain adequate indoor air quality and the second to the need to control indoor air temperature. The results, although only for a limited range of office buildings, indicated that the heat loss resulting from the use of windows, rather than other forms of heating control, could make a significant contribution to thermal performance and energy consumption. It is concluded that further work is required to develop methods for taking window use into account in building design and performance calculations.