The clinical, hematological and pathological features of the adult T-cell leukemia (ATL)-related diseases and their cell membrane phenotypes in the Nagasaki district, one of the ATL-endemic areas, were delineated to compare them with those of adult T-cell leukemia-lymphoma (ATLL). In addition, the participation of ATL virus (ATLV) in the ATL-related diseases was investigated on the basis of a study of ATLV-associated antigen (ATLA) and anti-ATLA antibody. ATLA and/or anti-ATLA antibody could be detected in the ATL-related diseases, including persistent T-lymphocytosis with morphologically abnormal T-lymphocytes, T-cell chronic lymphocytic leukemia (T-CLL) of helper T-cells, T-cell malignant lymphoma of the skin, some seronegative helper T-cell malignant lymphomas and monomorphic T-cell leukemia with round nuclei. These results suggest that those cases in the Nagasaki district, though clinically different from each other, might be the same T-lymphoproliferative disease of possible viral etiology associated with ATLV. On the other hand, persistent T-lymphocytosis seems to be an infectious condition caused by ATLV and partly a preclinical stage of ATLL in view of the clinicohematological features.