In epidemiological studies, increased indoor temperature--producing a lower relative humidity--is associated with low house dust mite (HDM) load. Twenty-eight dwellings were allocated for either intervention (12/15 completed) or control (11/13 completed). In the intervention group, participants were asked to increase the bedroom temperature by at least 3 degrees C compared to the self-assessed temperature of the previous winter. Dust samples were repeatedly collected from mattress and floor, and bedroom temperature and relative humidity were recorded hourly throughout one year. Dust was analysed for allergen (Der f 1 + Der p 1 + Der m 1 = Der 1) by ELISA and HDMs were counted. Changes in mite and in mite allergen concentration were the same in the control and intervention groups, and measured temperatures did not differ during intervention period in the groups (18 degrees C and 19 degrees C). Groups turned out not to be comparable with respect to initial (self-assessed) bedroom temperature (lowest in the intervention group). There was a significant seasonal variation, with doubled Der 1 concentrations in dust collected in July-November compared to January-May samples. No effect was obtained on mites or mite allergens, but this may be due either to a general lack of effect of increase in bedroom temperature, or to an insufficient increase in temperature in our intervention group. Seasonal variations in HDM and HDM allergens must be accounted for when data on exposure are interpreted.