Thirty-one adult patients with asthma caused by house-dust mites (HDM) were included in this placebo-controlled, double-blind study to evaluate the efficacy and safety of specific immunotherapy (SIT) with biologically standardized extracts of HDM. The specific diagnosis was confirmed by skin prick tests, specific IgE, and bronchial provocation tests with HDM allergens. The patients were randomized to receive active treatment with extracts of either Dermatophagoides pteronyssinus (Dpt) or D. farinae (Dfa) (Alutard SQ, ALK, Denmark) or placebo injections. Twenty-three patients completed the study. After 1 year of treatment, we found a clinically important and significant reduction in both asthma medicine consumption (inhaled steroids 38% and beta 2-agonists 46%) and symptom score (57%) in the actively treated group, but not the placebo group. These findings were confirmed by a significant decrease in skin and bronchial sensitivity to HDM in the active group. Additionally, there was a significant difference in the patients' scores for effect in favor of the actively treated group. Total IgE and specific IgE to HDM showed no significant changes before and after treatment for either group. Spirometric lung-function measurements showed a significant increase in forced expiratory volume in 1 s (FEV1) from 85% before to 89% of predicted values after treatment for the actively treated group. Peak-flow measurements at home showed no significant changes during the study. It is concluded that allergen SIT is an effective treatment in adult patients suffering from asthma due to HDM.