Sézary syndrome is a leukemic variant of cutaneous T-cell lymphoma characterized by the appearance of numerous CD4+ cells with cerebriform nuclei in the peripheral blood. Recent observations have suggested that Sézary cells lack CD7 molecules on their surface and are analogous to murine Th2 cells. It remains unclear, however, whether these two properties are actually common features of Sézary cells. We describe a case of Sézary syndrome in which more than 98% of the peripheral blood mononuclear cells expressed CD7 as well as a homogeneous T-cell receptor V alpha 2V beta 17, indicative of the expression of CD7 in the Sézary cells. Although the circulating Sézary cells continuously bore CD7 molecule on their surface throughout the patient's clinical course, the intensity of CD7 expression was variable in skin-infiltrating and in vitro cultured cells. Peripheral blood mononuclear cells from the patient proliferated well to a V beta 17-relevant superantigen (staphylococcal enterotoxin B) but not to irrelevant superantigens; produced interleukin-2, interferon gamma, and tumor necrosis factor-alpha, but not interleukin-4; and transcribed messenger RNA for interleukin-2 and interferon gamma but not interleukin-4 or interleukin-10. This represents an unusual case of a CD7+ Sézary syndrome with a cytokine profile characteristic of Th1 cells.