We performed a genotypic study on lymphoid cells from a patient with adult T-cell leukemia (ATL). Clonal proliferation of human lymphotropic virus type-1(HTLV-1)-infected helper T-cells was detected in the primary skin tumors, while no clonality of the virus-infected lymphocytes was observed in the peripheral blood. Following intensive chemotherapy, however, we detected the presence of two genotypically different HTLV-1-infected lymphocyte clones in different samples taken from the peripheral blood. These data demonstrated the exchange of the dominant neoplastic clone in the clinical course of ATL, suggesting the possibility that a certain clone among many repertoires of HTLV-1-infected T-cells has a capability of leukemogenesis, instead of the primary tumour cells.