Peripheral blood involvement by cutaneous T-cell lymphoma (CTCL) is typically assessed by flow cytometry (FC), and plays a critical role in diagnosis, classification and prognosis. Simplified strategies to detect tumor cells (Sezary cells) fail to exclude reactive subsets, while tumor-specific abnormalities are subtle and inconsistently present. We implemented a FC strategy to detect clonal Sezary cells based on the monotypic expression of one of two mutually exclusive T-cell receptor (TCR) β chain constant regions (TRBCs). Analysis of CD4-positive T-cell subsets and TCR variable β classes from healthy donors showed polytypic TRBC1 staining. Clonal Sezary cells were identified by TRBC1 staining in 56 of 111 (50%) samples from patients with CTCL, accounting for 7 to 18,155 cells/μL, and including 13 cases (23%) lacking tumor-specific immunophenotypic abnormalities. CD4-positive T-cell subsets from 86 patients without T-cell lymphoma showed polytypic TRBC1 staining, except for 5 patients (6%) with minute T-cell clones of uncertain significance (T-CUS) accounting for 53 to 136 cells/μL. Assessment of TRBC1 expression within a comprehensive single-tube FC assay effectively overcomes interpretative uncertainties in the identification of Sezary cells, without the need for a separate T-cell clonality assay.Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.