Background House dust mite numbers and the concentration of its main allergen, Der pl, depend on ambient temperature and humidity and have been reported to show a seasonal variation in homes in the United States. A study was designed to find out whether Der pl concentrations vary with the seasons in dust collected from homes in north west England. Methods The concentration of Der p1 was measured in dust, collected every three months from April 1990 to April 1991, from mattresses and from bedroom and living room carpets in 40 houses in the south Manchester area. Twenty four hour recordings of indoor relative humidity were made in 20 houses during the sampling day. Results Mean concentrations of Der pl from all three sources rose two to three fold in October. This was in contrast to the 1000 fold variation in concentrations of Der pl present between houses within each season's sample and less than the 10 fold change considered to be of clinical importance. The autumn increase was paralleled by a rise in humidity. There was no statistical correlation, however, between Der pl concentrations and relative humidity, house type, ventilation, or double glazing. Conclusions The results suggest that seasonal variations in exposure to Der pl exposure in north west England are small and unlikely to be of major clinical importance. The temperature and humidity data showed that the indoor environment remained relatively constant and conducive to mite growth throughout the year.