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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., (© 2016 The Authors. Cancer Medicine published by John Wiley & Sons Ltd.)