The various components making for severe intractable asthma were clinically and allergo-immunologically studied in 90 patients with bronchial asthma, by comparison between early onset and late onset groups. 1. In the early onset asthma group, cases with low serum IgE levels showed a stronger tendency toward severe intractable asthma. 2. Late onset asthma cases with negative skin tests and negative specific IgE antibodies to house dust tended more often to be severe intractable cases. 3. There was no correlation between sensitization by specific antigens (house dust and Candida), especially Candida, and a tendency toward severe intractable asthma. 4. Severe intractable asthma might be caused by bronchospasm in cases under 30 years of age, by bronchospasm plus hypersecretion in cases between 31 and 40 years of age, and by bronchospasm plus bronchiolar obstruction in cases over 40 years of age.