Background: Because there are currently many effective therapies available for Sé zary syndrome, close monitoring of disease progression is required in order for a clinician to know when to institute or change an intervention. It has been our clinical experience that changes in patients’ CD4+ CD26- T- cell populations of peripheral blood lymphocytes herald changes in their clinical status. Objective: Our purpose was to evaluate whether a change in patients’ CD4+ CD26- population of T cells presages a change in their clinical status. We also sought to investigate the association between a change in Tcell populations that are CD4+ CD7- , CD8+ , CD56+ , and the CD4 + /CD8+ T- cell ratio and a change in the patient’ s clinical status. Methods: We conducted a retrospective chart review analysis of 21 patients with Sé zary syndrome who had flow cytometry, usually including levels of CD4+ CD26- , CD4+ CD7- , CD8+ , CD56+ , and CD4+ /CD8+ ratios measured at two time periods, 12 weeks apart. Results: We report two cases in which changes in patients’ clinical status were preceded by several weeks by a change in their CD4+ CD26- level. We report weak associations between a decreasing CD4+ CD26- T- cell population, a decreasing CD4+ CD7- population, an increasing CD56+ population, and an improving clinical status. We also report stronger associations between both a decreasing CD8+ population and an increasing CD4+ /CD8+ ratio and a worsening clinical status. Limitations: The study was limited by the number of patients and the time period over which the study was conducted. In addition, varying configurations of CD4+ CD26- T- cell populations were observed that may have limited the utility of this measurement. Conclusions: Flow cytometry assays of patients’ blood and, in particular, measurement of the CD4+ CD26- population of lymphocytes over time may be a valuable tool for monitoring patients with Sé zary syndrome. There exist varying configurations of CD26 T lymphocytes that may cause differences in standards for what is considered positive and negative between observers. Further prospective analysis involving larger groups of patients is recommended. Background: Because there are currently many effective therapies available for Sé zary syndrome, close monitoring of disease progression is required in order in a clinician to know when to institute or change an intervention. It has been our clinical experience that changes in patients’ CD4 + CD26 - T-cell populations of peripheral blood lymphocytes herald changes in their clinical status. Objective: Our purpose was to evaluate whether a change in patients’ CD4 + CD26-population of T cells pres on a clinical association between a change in T cell populations that are CD4 + CD7-, CD8 +, CD56 +, and the CD4 + / CD8 + T-cell ratio and a change in the patient’s clinical status. Methods: We conducted a retrospective chart review analysis of 21 patients with Sé zary syndrome who had flow cytometry, usually including levels of CD4 + CD26-, CD4 + CD7-, CD8 +, CD56 +, and CD4 + / CD8 + ratios measured at two time periods, 12 weeks apart. Results: We report two cases in which changes in patients’ clinical status were preceded by several weeks by a change in their CD4 + CD26- level. We report a weak association between a decreasing CD4 + CD26- T-cell population, a decreasing CD4 + CD7- population , an increasing CD56 + population, and an improving clinical status. We also report that there is both a decreasing CD8 + population and an increasing CD4 + / CD8 + ratio and a worsening clinical status. Limitations: The study was limited by the number of patients and the time In addition, varying configurations of CD4 + CD26- T-cell populations were observed that may have limited the utility of this measurement. Conclusions: Flow cytometry assays of patients’ blood and, in particular, measurement of the CD4 + CD26- population of lymphocytes over time may be a valuable tool for monitoring patients with Sé zary syndrome. There exist varying configurations of CD26 T lymphocytes tha t may cause differences in standards for what is considered positive and negative between observers. Further prospective analysis involving larger groups of patients is recommended.