Adult T-cell leukemia/lymphoma (ATLL) is a lymphoproliferative neoplasm of helper T lymphocytes caused by human T-cell leukemia virus type-1 (HTLV-1). The disease was first described in Kyushu, in southwestern Japan, and most frequently occurs in endemic areas, such as Japan, the Caribbean basin, West Africa, Brazil, and northern Iran. ATLL is essentially a disease of adults, characterized clinically by generalized lymphadenopathy, hepatosplenomegaly, skin lesions, and hypercalcemia. The prognosis of most patients is quite poor, with a median survival time of only 13 months, even if multiagent combination chemotherapy is given. In the present study, flow cytometric immunophenotyping with CD3 gating was performed on 30 samples from 26 patients who had been given a diagnosis of ATLL. The records of these patients also were reviewed retrospectively. In 14 of the 30 samples, an abnormal CD3(low) T-cell population was distinguishable from the normal T-cell populations by flow cytometric analysis. Herein we report a novel strategy for flow cytometric immunophenotyping of ATLL facilitated by CD3(low) gating.