Clinical and hematologic studies of 16 adult patients whose leukemic cells had T-cell markers are reported from Japan, where the incidence of various lymphoproliferative diseases differs considerably from that in Western countries. Leukemic cells were studied by cytotoxicity tests with specific antisera against human T (ATS) and B cells (ABS) in addition to the usual T- and B-cell markers (E rosette, EAC rosette, and surface immunoglobulins). Characteristics of the clinical and hematologic findings were as follows: (1) onset in adulthood; (2) subacute or chronic leukemia with rapidly progressive terminal course; (3) leukemic cells killed by ATS and forming E rosettes; (4) leukemic cells not morphologically monotonous and frequent cells with deeply indented or lobulated nuclei; (5) frequent skin involvement (9 patients); (6) common lymphadenopathy and hepatosplenomegaly; (7) no mediastinal mass; and, the most striking finding, (8) the clustering of the patients’ birthplaces, namely, 13 patients born in Kyushu. The relationship between our cases and other subacute or chronic adult T-cell malignancies such as chronic lymphocytic leukemia of T-cell origin, prolymphocytic leukemia with T-cell properties, Sézary syndrome, and mycosis fungoides is discussed.