The molecular and genetic factors induced by human T-lymphotropic virus type-1 (HTLV-1) that initiate adult T-cell leukemia/lymphoma (ATLL) remain unclear, in part from the lack of an animal model that accurately recapitulates leukemogenesis. HTLV-1-infected humanized nonobese diabetic severe combined immunodeficiency (HU-NOD/SCID) mice were generated by inoculation of NOD/SCID mice with CD34(+) hematopoietic progenitor and stem cells (CD34(+) HP/HSCs) infected ex vivo with HTLV-1. HTLV-1-HU-NOD/SCID mice exclusively developed CD4(+) T-cell lymphomas with characteristics similar to ATLL and elevated proliferation of infected human stem cells (CD34(+)CD38(-)) in the bone marrow were observed in mice developing malignancies. Purified CD34(+) HP/HSCs from HTLV-1-infected patient peripheral blood mononuclear cells revealed proviral integrations suggesting viral infection of human bone marrow-derived stem cells. NOD/SCID mice reconstituted with CD34(+) HP/HSCs transduced with a lentivirus vector expressing the HTLV-1 oncoprotein (Tax1) also developed CD4(+) lymphomas. The recapitulation of a CD4(+) T-cell lymphoma in HU-NOD/SCID mice suggests that HSCs provide a viral reservoir in vivo and act as cellular targets for cell transformation in humans. This animal model of ATLL will provide an important tool for the identification of molecular and cellular events that control the initiation and progression of the lymphoma and potential therapeutic targets to block tumor development.