Dermatophagoides farinae (D.F.) is considered by many to be one of the main allergens in house dust. The present report is a four-year study with an extract of D.F. in 518 patients with extrinsic asthma. The extract was clinically effective in 315 out of 474 patients (66.5%) after the first course of therapy (20 weeks). It was even better (P less than 0.001) after more than two courses (76.5%). About 10% of the patients become almost entirely asymptomatic. The individuals whose skin reactions decreased after treatment showed the best results (P less than 0.05). After therapy the skin reaction and degranulation of mast cells were inhibited. Side effects were mild and transient. The results suggest that immunotherapy with D. farinae might be an acceptable measure of treatment for many patients with extrinsic asthma.