The mechanisms of extracorporeal photochemotherapy (ExP) therapeutic activity in cutaneous T-cell lymphomas (CTCLs) are not yet well understood, even though it has been suggested that a major mechanism may be induction of apoptosis. In vitro studies demonstrate that UVA-induced apoptosis is mediated by CD95-Fas expression and inhibited by Bcl-2 up-regulation and that UVA irradiation is able to down-regulate Bcl-2 expression. High-resolution multiparameter flow-cytometric analyses were used to evaluate Bcl-2/CD95-Fas expression on phenotypically identifiable circulating clonal T cells from 7 patients with CTCL (4 with Sézary syndrome and 3 with mycosis fungoides with peripheral involvement) before and during ExP, in an attempt to ascertain whether Bcl-2/CD95-Fas status can be related to the hematologic response. A Bcl-2 normal phenotype before ExP or a normalization in Bcl-2 expression during ExP were related to a better clinical response, whereas a persistent Bcl-2 high expression was a negative prognostic factor. On the other hand, no response was found in patients with a CD95-Fas-negative phenotype, whereas the expression of CD95-Fas was associated with hematologic remission. Although further studies are needed to confirm these preliminary results, this study suggests that Bcl-2 and CD95-Fas expression could be evaluated, together with the other known clinical and immunologic factors, as additional parameters related to clinical response in patients with CTCL undergoing ExP. (J Am Acad Dermatol 2001;44:40-7.)