Peak expiration flow records from patients allergic to house-dust mites (Dermatophagoides spp.) may show a characteristic variation from week to week due to the general life cycle of these mites in dwellings. This was demonstrated from the combined records of 10 patients recorded in their own homes, covering a period of 30 consecutive weeks and comprising 973 peak flows. The levels of house-dust mites were predicted from published data for floor-dust samples from Danish dwellings. Peak flow increased or decreased in accordance with weekly changes in the concentration of mites, rather than as an immediate reaction to the current concentration of live, active mites. A dose-response relation was demonstrated. This suggests that the patients' peak flow variations might be linked to molting in mites. As such, peak flow measurements have a potential as a tool for the specific diagnosis of, monitoring of, and research in asthma caused by domestic mites.