Allergen injection therapy may improve nonallergic bronchial hyperresponsiveness, but results at the moment are less than convincing. The present study was conducted to evaluate the effect of immunotherapy on the degree of nonspecific bronchial hyperresponsiveness in patients with allergic bronchial asthma (BA) and/or allergic rhinitis (AR). Methacholine challenge bronchial provocation test, allergic skin test, serum IgE and peripheral blood eosinophil counts were performed before and after 12 months or more of immunotherapy. The improved group, as determined by a shift of at least two doubling concentrations of methacholine, was 75% of AR (n=16), 41.7% of BA (n=24) and 53.8% of BA+AR (n=13). The geometric mean of the methacholine provocational concentration (PC₂₀) changed from 3.40 to 14.36 mg/ml (P < 0.05) in AR, from 0.73 to 1.04 mg/ml in BA (not significant), and from 1.43 to 5.07 mg/ml (P <0.05) in BA+AR. In conclusion, nonspecific bronchial hyperresponsiveness was improved by immunotherapy in three quarters of the allergic rhinitis cases and in about a half of the allergic bronchial asthma patients, which suggests that immunotherapy might be helpful at preventing the development of bronchial hyperresponsiveness in allergic rhinitis patients, and that it does not improve bronchial hyperresponsiveness in about a half of allergic bronchial asthma patients.