Background: The relationship between exposure to house dust mite (HDM) allergens and prevalence of sensitization to these allergens in patients with asthma has been confirmed in many studies. Mite population growth is regulated by humidity. Reducing humidity and removing allergen by efficient vacuuming should control mite allergen and reduce symptoms. Objective: We sought to investigate the effect of mechanical ventilation and high-efficiency vacuuming on HDM numbers and Der p 1 concentrations in the homes of mite-sensitive asthmatic subjects and to evaluate the effect of any reductions on symptoms. Methods: The homes of 40 HDM-sensitive asthmatic subjects were randomized to receive (1) mechanical ventilation and a high-efficiency vacuum cleaner (HEVC); (2) mechanical ventilation alone; (3) an HEVC alone; and (4) no intervention. Homes and patients were monitored for 12 months. Change in absolute humidity, mite numbers, Der p 1 concentrations, lung function, bronchial hyperresponsiveness, and symptom scores were analyzed. Results: Homes with mechanical ventilation achieved significantly lower humidity levels than those without (P < .001), with an associated reduction of mite numbers (P < .05) and Der p 1 concentrations (P < .001 [in nanograms per gram], P = .006 [in milligrams per square meter]) in bedroom carpets and some other mite sources in the ventilated areas of the homes. The addition of a vacuum cleaner enhanced this effect. There was a trend for an improvement in histamine PC₂₀ (P = .085) in the patients whose homes were ventilated. Conclusion: The use of a mechanical ventilation system in suitable homes resulted in some reduction in numbers of HDM and Der p 1 concentrations. The addition of an HEVC slightly enhanced the effect but not sufficiently to see an improvement in symptoms. (J Allergy Clin Immunol 2000;105:75-82.)