In Sweden, the energy crisis in the early seventies resulted in attempts to lower the air change rate in buildings to reduce energy consumption. For many building categories, this lead to a deterioration of the indoor climate or problems with moisture and mould growth, Today, many residents demand higher ventilation rates, often the motivation is concerns about health and comfort. In this paper is presented results of measured air change rates in Swedish dwellings during the period 1974-88. For some residential building categories, the average air change rate is much smaller than that prescribed by the Swedlsh Building Code, 0.5 air changes per hour ( ACH ) . About half the apartments and houses with natural ventilation constructed after 1960 have a rate of air exchange that is 0.3 ACH, or lower. This is also the case for about fifteen per cent of the houses with exhaust air ventilation. For buildings with mechanical ventilation, it has been investigated what are the differences in ventilation rates for apartments in the same building and for groups of identical houses after the final installation and control of the ventilation system. In both cases the scatter around the average is about 20 % .