Adult T-cell leukemia-associated antigen (ATLA), a human retrovirus-associated antigen, and anti-ATLA antibodies were examined in 52 cases of T-cell malignancies in the Nagasaki district in southwestern Japan, which is known to be an ATL endemic area. These T-cell malignancies included 27 cases of ATL, 2 cases of T-cell chronic lymphocytic leukemia (T-CLL), 18 cases of T-cell lymphoma, and 5 cases of mycosis fungoides. Using the immunofluorescence method, ATLA-positive cells were detected in short-term mass culture of mononuclear cells from 22 of the 27 ATL patients, both T-CLL patients, 17 of 18 T-cell lymphoma patients, and 3 of the 5 mycosis fungoides patients. In an ATL patient in whom a high percentage of ATLA-positive cells were detected, many type C virus particles were observed in the extracellular space of the cultured mononuclear cells with an electron microscope. Sera from all 27 of the ATL patients, the 2 T-CLL patients, 15 of the 18 T-cell lymphoma patients, and 4 of the 5 mycosis fungoides patients were anti-ATLA antibody positive. These results indicate the possible participation of the retrovirus, ATL virus (ATLV), in these T-cell malignancies in ATL-endemic areas.