Twenty-seven asthmatic children allergic to dog were included in the study. Their allergy to dog was confirmed by positive results from the skin prick, radioallergosorbent and provocation tests. The subjects were randomly assigned to active and placebo groups. Fifteen subjects received immunotherapy with a commercial standardized aluminum hydroxide-bound dog dander extract and 12 subjects received placebo injections containing histamine. After one year's therapy the conjunctival sensitivity to dog dander extract had decreased significantly (p less than 0.001) in the active treatment group compared to the placebo group. The hyposensitization effect was already observable after six to eight months of treatment. The decrease in bronchial sensitivity was less marked than that in conjunctival sensitivity and statistically not significant. Immunotherapy was observed to be safe and effective. No systemic adverse reactions occurred and local reactions were mild.