['Clonal expansion of lymphocytes may account for asymptomatic lymphocytosis. Unlike monoclonal B lymphocytosis, the detection of T-cell clones is difficult using immunophenotype-based assays. This study aimed to evaluate PCR-based clonality assays for the identification of T-cell clones among patients with unexplained lymphocytosis.', 'We incorporated the BIOMED-2 multiplex PCR into the investigation of peripheral lymphocytosis. Clonality analysis was performed with three reactions for TCRβ and two reactions for TCRγ gene rearrangements. The analysis was performed with blood specimens from a total of 150 adult patients who presented with incidental lymphocytosis.', 'PCR assays were validated using confirmed T-cell malignancies and the detection sensitivity was determined at a level of 2%. Using the TCRβ and TCRγ combination, 25 (16.6%) of 150 patients were found to have clonal TCR arrangement. Patients who harboured clonal T cells presented with a mild to moderate absolute lymphocytosis, with a median lymphocyte count of 4.5 × 10(9)/l (range 3.7-9.8 × 10(9)) and the absence of other haematological abnormalities. Immunophenotyping confirmed T-cell lymphocytosis with an increase in CD8 T cells in the majority of patients.'] Is t-cell clonality detected in a high frequency among patients with incidental lymphocytosis by PCR assays for TCR gene rearrangements?