Serum IgE level was measured by the method of radioimmunosorbent test (RIST) in 353 subjects including 183 patients with bronchial asthma. One hundred and twenty patients with other pulmonary diseases and allergic diseases, and 50 normal adults for the control. Also, the level of specific IgE antibody to house dust was measured by the method of radioallergosorbent test (RAST) in patients with bronchial asthma. In patients with bronchial asthma, the IgE level was evaluated according to the following conditions, i.e., types by Swinfold's classification, age groups, seasons, peripheral blood counts of basophils and eosinophils and effects by immunotherapy. At the same time, the changes of specific IgE antibody level during the periods of house dust desensitization therapy was evaluated. The relusts were as follows: 1) The normal value of serum IgE level was 189±156 u/ml (mean±one standard deviation) and bronchial asthma patients showed 1121±1147 u/ml, which was remarkably high. And, significantly high values were observed in patients with bronchial asthma, allergic rhinitis, drug allergy, diffuse interstitial pneumonitis and parasite diseases compared to normal control. In chronic bronchitis, pulmonary tuberculosis, pulmonary sarcoidosis, IgE level was about the same as control. 2) Serum IgE level was found to reduce with aging. 3) The seasonal changes of serum IgE level was not significant. 4) Serum IgE level was significantly higher in patients with atpic bronchial asthma than infectious type of bronchial asthma according to Swinfold's classification. 5) The peripheral blood counts of basophils and eosinophils did not correlate significantly with serum IgE level. 6) Serum IgE level after the long term house dust desensitization therapy did not change, but specific IgE antibody was found to be decreased in 2 out of 5 bronchial asthma patients with more than 2+RAST score. 7) In intractable asthmaticus the significant relationship was observed between the clinical improvement by the continuous low dose administration of 6-Mercaptopurine and the decrease of serum IgE level, but the corticosteroids therapy did not affect on the IgE level.