In the 1990's, concern about global warming has resulted in a resurgence of interest in naturally ventilated offices. The Belgian climate is particularly well adapted to apply cooling by natural ventilation. Indeed, except for a few hours a year, outdoor air temperature is lower than indoors. Lots of office buildings have no atrium or chimney to benefit from any stack ventilation. But natural ventilation can nevertheless be organized with only frontage windows either by single-sided ventilation or by cross ventilation. Simulations with the software TAS were made to appraise those strategies. Zone air flows and needs for cooling in an office building in various situations, as well as the impact of the wind orientation were analysed. The impact of size and location of window apertures on ventilation rates was also studied.