Background: A 5-year follow-up study was conducted to investigate the duration of the effects of a 3-year course of immunotherapy with standardized cat or dog extracts in 32 children and adults with asthma caused by animal dander. Methods: Thirty of the subjects could be reached with a questionnaire, 19 underwent bronchial allergen and histamine challenges, and four had only a histamine challenge. Specific IgE and IgG₄ levels in serum were measured in those who underwent challenges. Results: Almost all subjects (26 of 30) reported no change (17 subjects) or increased tolerance (9 subjects) on exposure to cats or dogs. In contrast, 17 of the 19 who underwent allergen challenges had increased allergen sensitivity compared with when therapy was stopped (p < 0.01), and the results were no longer significantly different from before therapy was started. Mean provocative concentration of histamine causing a 20% fall in peak expiratory flow was, however, still higher than before therapy in the cat immunotherapy group (p < 0.01) and had not changed significantly during the follow-up period. In the dog immunotherapy group there was no significant change during or after therapy. Specific IgG₄ had decreased, and specific IgE in serum had remained low and was comparable to the levels measured at the end of the study period. Conclusions: Five years after stopping immunotherapy, objectively measured bronchial allergen sensitivity had increased and had approached pretreatment conditions. Asthma symptoms, according to patients' subjective evaluations, had continued to be mild in most patients, and bronchial histamine sensitivity had remained stable. These observations could reflect remaining effects of immunotherapy or the natural history of mild asthma. (J ALLERGY CLIN IMMUNOL 1995;96:879-85.)