Thirteen house dust mite-allergic young asthmatics were entered into a double-blind, crossover clinical trial to compare "in use" with "non-use" of a laminar flow air cleaner bed attachment. The study design encouraged maintenance of symptom control by adjustment of the dose of medications used. There was a significant reduction in the amount of medications required by the patients during the trial period that the air cleaner was in use. Histamine airway responsiveness was significantly decreased for a group of eight of the subjects tested at the end of a 3-month open trial.