Repeated surveys on occupant behaviour and indoor climate were carried out in Danish dwellings from September to October 2006 and again from February to March 2007. The summer survey comprised 933 respondents and the winter survey 636 respondents. The surveys were carried out by sending 5000 invitations to a sample of the Danish housing stock. The addresses were obtained from a Danish register along with information on physical properties of each dwelling. Meteorological data was obtained from the Danish Meteorological Institute. The outdoor temperature, solar radiation and type of supplementary heating had the largest effect on the proportion of dwellings with the heating on. The outdoor solar radiation and the perceived illumination had the largest effect on the proportion of dwellings with lights on. The most frequent reason listed for opening the windows in summer and winter was to obtain more air movement.