Eighteen patients with DPB and 15 with IIP were studied on the relationship between the diseases and ATL. Anti-ATL antigen (ATLA) antibody and its related reactions were examined using immunofluorescence and Western blotting techniques. Five of 18 DPB patients (28%) were anti-ATLA antibody positive, and were diagnosed as ATL because of the appearance of ATL cells after the chronic DPB clinical course. No IIP patient was diagnosed as having ATL, however, a patient's serum converted to anti-ATLA antibody positive from ATLA related reaction positive when tested with immunofluorescence. ATLA related reaction with diffuse reaction patterns in both MT-1 and MT-2 cells was detected in 7 of 18 sera in patients with DPB and in 5 of 15 IIP sera. A diffuse pattern only in MT-2 cells was found in 2 cases of DPB and 3 of IIP. Positive percentage of these anti-ATLA antibody and ATLA related reactions in DPB and IIP patients was significantly higher than that in controls. These results suggest the existence of a specific clinicopathological state namely "HTLV-1 associated bronchiolo-alveolar disorder (HABA)" in some of DPB and IIP.