A 64 year-old Japanese man who developed acute monoblastic leukemia during the course of adult T-cell leukemia/lymphoma (ATL) was studied. Leukemic cells in the peripheral blood and bone marrow were monoblasts positive for alpha-naphthol butyrate esterase (alpha-NBE) staining, CD11c and CD36 antigens, whereas tumor cells in the pleural effusion were ATL cells positive for CD2, CD4, CD25, CD29 and CD45RA antigens. These two malignant cells had different chromosomal abnormalities. Monoclonal integration of human T-cell leukemia virus type I (HTLV-I) proviral DNA and T-cell receptor C beta gene (TCR C beta) rearrangement were detected in the ATL cells, but not in the leukemic monoblasts. By polymerase chain reaction (PCR) in the peripheral blood mononuclear cells (CD11c+ 98%, CD2+ 4%, CD20+ 0%) not containing ATL cells, the presence of the gag region of HTLV-I was confirmed. These facts indicate that a double positive T cell (CD29+, CD45RA+) was possibly the target cell for HTLV-I infection and that HTLV-I was not directly related to the oncogenesis of the monocyte lineage in the present case, even if it did infect the monocytes. However, there is still an outside possibility that HTLV-I induced acute monoblastic leukemia indirectly.