We evaluated 44 peripheral blood, bone marrow, and lymph node specimens from 23 patients with the clinical diagnosis or suspicion of cutaneous T-cell lymphoma by 4-color flow cytometry for the presence of altered CD3 expression and compared the results with light microscopic evidence of involvement by mycosis fungoides (MF) or Sézary syndrome (SS). In 19 specimens (15 peripheral blood, 3 lymph node, 1 bone marrow) from 6 patients, we noted reduced CD3 expression compared with normal T lymphocytes. Diminished CD3 expression correlated with morphologic evidence of MF/SS cells in all cases, although the enumeration of MF/SS cells differed. Comparison of the enumeration of MF/SS cells in the peripheral blood by CD3+CD7- phenotyping with CD3dim phenotyping suggested that CD3dim phenotyping was the most accurate method for counting MF/SS cells. Therefore, diminished CD3 expression as shown by flow cytometry may be particularly useful for detecting and enumerating MF/SS cells in hematopoietic tissue of patients with cutaneous T-cell lymphoma.