Objective: To analyze the distribution and clonality of TCR Vβ subfamily T cells in hairy cell leukemia (HCL) and common-acute lymphoblastic leukemia (c-ALL). Methods:Peripheral blood mononuclear cell samples from 3 cases of HCL and 1 case of c-ALL were investigated for analysis of complementarity determining region 3 (CDR3) size of T cell receptor Vβ repertoire using reverse transcriptasepolymerase chain reaction (RT-PCR). The products were further analyzed by genescan to identify T cell clonality.Results: Some Vβ subfamily PCR products from 4 patients contained monopeak (monoclone) or a dominant peak (oligoclone). In contrast, multipeak (polyclone) distributions were found in all Vβ subfamily PCR products from normal control cases. Conclusion: T cell clonal expansion may be found in HCL and c-ALL cases that may indicate a host response directed against leukemia related antigen. In addition, it may be useful to detect the minimal residual disease.