Closed courtyards adjacent to houses are a common element in the architecture of arid and semi-arid zones. While serving a variety of social aspects of life in these areas, it is also believed that these courtyards provide a pleasant place to spend time in, and that they also improve the micro-climate inside the building by providing a source of cooler air for ventilation. A study was undertaken to examine the thermal behaviour of closed, non-shaded courtyards. It was found that in summer, most of the time the courtyard is warmer than the ambient air temperature near the house, day and night. In a few cases the temperature differences between the ambient air and the air inside the courtyard were as high as 7°C. In winter, the temperatures of the courtyard were somewhat moderated compared to the ambient, but even then in most cases the courtyard was warmer than the ambient air. It seems that for courtyards in hot arid zones to operate as claimed they must be well shaded and well ventilated; when they are not, they increase the heat stress in and around the building.