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 non-malignant (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 ten HTLV-1-uninfected subjects (UI), 54 HTLV-1infected 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 non-progressing 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.