House dust mite (HDM) allergen exposure and its relation to HDM allergy and asthma was assessed in a case-control study conducted over three seasons in 74 Sydney schoolchildren, 33 of whom were allergic to HDM and 12 of whom had current asthma. In each season histamine inhalation tests and skin prick tests were performed, symptom questionnaires were administered, and dust samples were collected. The mean concentrations of HDM allergen (in micrograms of Der p 1 per gram of fine dust) were: bed, 38.9 (95% confidence interval [CI], 31.8 to 47.5); bedroom floor, 22.4 (95% CI, 18.3 to 27.5); and lounge room floor, 13.7 (95% CI, 10.7 to 17.6). The mean of the highest allergen concentration in each house was 51.0 (95% CI, 43.2 to 60.1). All but two subjects had at least one site in all seasons with an HDM allergen concentration greater than 10 micrograms/gm, the proposed threshold for asthma symptoms. Subjects with allergy to HDM, symptoms of asthma, or airway hyperresponsiveness did not have higher HDM allergen concentrations in their house. In this study we were unable to test hypotheses concerning proposed thresholds for risk of sensitization and for risk of asthma symptoms because virtually all subjects were exposed to HDM allergen levels above the proposed thresholds.