To elucidate the role of lymphocytes in asthmatics, lymphocyte subpopulations were analyzed using monoclonal antibodies (CD2, CD3, CD4, CD8, CD16, CD19, CD25, and HLA-DR, all from Leu series) and flow cytometry (FACScan, Beckton-Dickinson). Blood samples were obtained when there was no attack unless otherwise stated. Patients were defined as intractable when they had been taking more than 5 mg of prednisolone for at least one year. (1) Non-intractable asthmatics had greater ％ of CD16＋NK cells as compared with agematched normal subjects. (2) No differences were observed between RAST or intradermal test-positive and negative patients, although the former had significantly higher serum IgE levels than the latter. (3) Intractable asthmatics showed an increased proportion of IL－2R＋lymphocytes as compared with normal subjects. (4) Patients on attack also had a higher ％ of IL－2R＋lymphocytes than patients without attack. These findings suggest that NK cells play an important role in bronchial asthma, and that activeted lymphocytes are involved in the pathogenesis of intractable asthma and asthmatic attack.