The aim of the study was to describe the distribution of house dust mite (HDM) allergens within homes of three-year-old children, to identify factors responsible for its variation and to test the hypothesis whether the content of HDM allergens exceeding 2 [mu]g/g dust may be regarded as a risk level of sensitization possibly affecting respiratory health in early childhood. House dust samples were collected in 279 dwellings from the mattresses, children's bedroom and kitchen floors. In the laboratory, dust samples were analyzed for Der f 1 and Der p 1 using monoclonal antibody enzyme-linked immunosorbent assays. At the time of the house dust collection, interviews with mothers on the household characteristics and child's respiratory health were performed. Respiratory outcome variables included running or stuffed nose, cough, barking cough, difficult (puffed) breathing and wheezing or whistling in the chest irrespective of respiratory infection. For each of the symptoms the number of the episodes and duration in days over the past six months were recorded in the questionnaire. In the multivariate Poisson regression analysis, a set of potential confounders has been taken into account such as gender of child, season, maternal education, maternal atopy, older siblings and environmental tobacco smoke. The adjusted rate ratio for episodes of running nose among those exposed to higher HDM exposure was 1.09 (95% confidence interval (CI): 0.98-1.20), for episodes of cough 1.11 (95% CI: 0.99-1.24), barking cough 1.44 (95% CI: 0.99-2.09) difficult breathing 1.31 (95% CI: 0.91-1.92) and for wheezing 1.42 (95% CI: 1.00-2.02). In contrast, rate ratios for the duration of all respiratory symptoms were significantly higher in children exposed to higher HDM level. The corresponding rate ratios for duration of symptoms (in days) were 1.10 (95% CI: 1.06-1.14), 1.06 (95% CI: 1.02-1.11), 1.64 (95% CI; 1.42-1.90), 2.05 (95% CI: 1.79-2.35) and 1.40 (95% CI: 1.21-1.62). The children with positive history of maternal atopy showed a higher risk of wheezing than those without the maternal atopy. The rate ratios for number of wheezing episodes in children of mothers without atopy was insignificant 1.11 (95% CI: 0.94-1.30) while in the group with maternal atopy the risk ratio amounted to 1.47 (95% CI: 1.09-1.96); the corresponding rate ratio estimates for the duration of wheezing were 1.12 (95% CI: 1.04-1.20) and 1.46 (95% CI: 1.29-1.66). The data support the view that exposure to higher level of HDM allergens may affect susceptibility of the bronchi to environmental factors and increase the burden of respiratory diseases in early childhood. (Contains 12 tables and 3 figures.)