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Adult T-cell leukemia/lymphoma ( ATL), a human T-lymphotropic virus type 1 ( HTLV-1)-associated disease, has a highly variable clinical course and four subtypes with therapeutic and prognostic implications. However, there are overlapping features between ATL subtypes and between ATL and nonmalignant (non- ATL) HTLV-1 infection complicating diagnosis and prognostication. To further refine the diagnosis and prognosis of ATL, we characterized the immunophenotype of HTLV-1-infected cells in ATL and non- ATL. A retrospective study of peripheral blood samples from 10 HTLV-1-uninfected subjects ( UI), 54 HTLV-1-infected patients with non- ATL, and 22 with ATL was performed using flow cytometry. All patients with ATL had CD4+ CCR4+ CD26− immunophenotype and the frequency of CD4+ CCR4+ CD26− T cells correlated highly significantly with the proviral load in non- ATL suggesting CD4+ CCR4+ CD26− as a marker of HTLV-1-infected cells. Further immunophenotyping of CD4+ CCR4+ CD26− cells revealed that 95% patients with ATL had a CD7− (≤30% CD7+ cells), whereas 95% HTLV+ non- ATL had CD7+ (>30% CD7+ cells) immunophenotype. All patients with aggressive ATL had a CCR7+ (≥30%), whereas 92% with indolent ATL and 100% non- ATL had a CCR7− (<30%) immunophenotype. Patients with nonprogressing indolent ATL were CD127+ but those with progressive lymphocytosis requiring systemic therapy had a CD127− (≤30%) immunophenotype. In summary, HTLV-1-infected cells have a CD4+ CCR4+ CD26− immunophenotype. Within this population, CD7− phenotype suggests a diagnosis of ATL, CCR7+ phenotype identifies aggressive ATL, while CCR7− CD127− phenotype identifies progressive indolent ATL. [ABSTRACT FROM AUTHOR]