Background. Sensitization and exposure to mite allergens is a major risk factor for asthma. Little is known about the rate of build-up of allergens in the mite microhabitats. Objectives. To investigate the rate of increase in mite allergen levels in new mattresses. Methods. Der p 1 was measured in the dust samples collected from six identical new single mattresses over a period of 2 years. Results. Der p 1 increased significantly at 4 months as compared with baseline level (P <0.01), but no difference was found between the concentrations at 4, 8, 12 and 24 months. There was a significant correlation between Der p 1 concentration in mattresses at 4, 8, 12 and 24 months and Der p 1 levels in the bedroom carpet at the beginning of the study. Conclusions. New mattresses can become a significant source of exposure to mite allergens after a short period of time (<4 months). There is little justification for advising mite sensitive patients to replace their mattresses as a part of avoidance regime.