OBJECTIVE The aim of the study was to seek for a possible association between the incidence of upper respiratory tract infections and air temperature and humidity in the home. METHODS Recordings of temperature and relative humidity were made in living rooms and children's bedrooms over a six month period and related to incidence of upper respiratory tract infection. METHODS The study was carried out in one general practice of 10,000 patients. METHODS 297 children aged 24-59 months were studied, selected in random order from the practice age-sex register. RESULTS Temperature and humidity recordings were made with thermohygrograph recorders over six days. Upper respiratory tract infections were recorded (a) retrospectively over the previous 12 months, and (b) during the study period. Past history of acute otitis media and recent family history of respiratory infection were also obtained. No significant association was found between the variables, although the bedrooms of children with reported upper respiratory tract infections were cooler overnight than those of non-infected children (mean difference 0.8 degrees C, 95% confidence limits 0.7 degrees C). No association was found between reported or recorded upper respiratory tract infections and age or type of home, family size, level of occupancy, social class, or smoking habits. Only 15 children (5%) were identified by their parents as having had asthma, but 58 (19.5%) had had a "wheezy chest". A greater proportion of children who wheezed slept in cooler bedrooms, had gas fires rather than central heating, and had more smokers in the house. CONCLUSIONS No association between upper respiratory tract infection and domestic temperature or humidity levels could be shown in this study. Since dampness is repeatedly presented as a health risk, further study is required.