In order to clarify the immunological characteristics of late onset asthma, 106 cases of asthma in patients over 40 years old were divided into two groups: the early onset asthma group at an age less than 40 years old and the late onset asthma group which was 40 or more years old at onset. The results obtained in each group were compared. 1) Asthma patients with a positive family history or with other allergic diseases in the past history were significantly less in the late onset asthma group compared with the early onset asthma group. 2) An increased eosinophil count in the peripheral blood was observed in both groups with no significant difference. 3) There were more patients with low IgE level (less than 300IU/ml) in the late onset asthma group than in the early onset asthma group. 4) In skin tests to 8 common allergens, the incidence of positive immediate reaction to house dust and aspergillus was low in the late onset asthma group. There was no difference in late or delayed reactions between the two groups. 5) The number of patients with positive serum specific IgE antibody to house dust or mites were few in the late onset asthme group. 6) In the bronchial provocation test with house dust, the proportion of positive response was low in the late onset asthme group. 7) The attack symptoms of asthma were severe and perennial in the late onset asthma group compared with those of the early onset asthma group.