The authors report the flow cytometric detection of neoplastic T cells in the peripheral blood of four out of five (80%) patients with peripheral blood involvement with mycosis fungoides (Sezary syndrome) based on the levels of T-cell receptor expression as measured by CD3 and TCR-alpha beta staining. Antigen receptor expression was abnormal in terms of increased density of surface CD3 or TCR-alpha beta per cell. Other immunophenotypic abnormalities were present in three of these patients. However, in one patient abnormal T-cell receptor expression was the only immunophenotypic evidence of neoplasia, although morphologically abnormal lymphocytes were present and a T-cell clone was detected by polymerase chain reaction (PCR). In another patient, the authors were able to detect development of a new, more aggressive neoplastic T-cell population based on levels of T-cell receptor expression. Levels of T-cell receptor expression may be of diagnostic utility in the evaluation of peripheral blood for the presence of neoplastic T-cell populations.