To study the pathogenesis of HTLV-Ⅰassociated bronchiolo-alveolar disorder (HABA), Western blot analysis was performed using MT-2 cell lysate antigens on the serum of 6 patients with HABA, 7 patients with adult T cell leukamia (ATL), 14 asymptomatic carriers and 9 healthy controls. Five(83%) of the 6 HABA patients were positive for anti-HTLV-Ⅰ IgA antibodies, although all 7 ATL patients and 11 of the 14 asymptomatic carriers were negative. All HABA and ATL patients and asymptomatic carriers were positive for anti-HTLV-Ⅰ IgG antibodies. Four(67%) of the 6 HABA patients, 2(29%) of the 7 ATL patients and 9(64%) of the 14 asymptomatic carriers were positive for anti-HTLV-Ⅰ IgM antibodies. Statistically there was little difference in the incidences of anti-HTLV-Ⅰ IgG and IgM antibodies among HABA and ATL patients and asymptomatic carriers, but the HABA patients showed a significantly higher incedence of IgA antibody than the ATL patients and asymptomatic carriers. Since specific IgA antibodies are produced in the local infected mucosa and part of the IgA antibodies are transferred into the serum, in conclusion, the anti-HTLV-Ⅰ IgA antibodies in HABA patients reflect the localization of HTLV-Ⅰ in lungs which play an important role in the pathogenesis of HABA.