Adult T-cell leukemia (ATL) is a peripheral CD4+ T-cell neoplasm caused by human T-cell leukemia virus type 1 (HTLV-1). Despite several investigations using human specimens and mice models, the exact origin of ATL cells remains unclear. Here we provide a new insight into the hierarchical architecture of ATL cells. HTLV-1-infected cells and dominant ATL clones are successfully traced back to CD45RA+ T memory stem (TSCM) cells, which were recently identified as a unique population with stemlike properties, despite the fact that the majority of ATL cells are CD45RA- CD45RO+conventional memory T cells. TSCM cells from ATL patients are capable of both sustaining themselves in less proliferative mode and differentiating into other memory T-cell populations in the rapidly propagating phase. In a xenograft model, a low number of TSCM cells efficiently repopulate identical ATL clones and replenish downstream CD45RO+ memory T cells, whereas other populations have no such capacities. Taken together, these findings demonstrate the phenotypic and functional heterogeneity and the hierarchy of ATL cells. TSCM cells are identified as the hierarchical apex capable of reconstituting identical ATL clones. Thus, this is the first report to demonstrate the association of a T-cell malignancy with TSCM cells.