OBJECTIVE To determine if there are differences in the expression of programmed death-1 (PD-1) between SS and MF, and in particular erythrodermic MF (E-MF). PD-1 is a marker of follicular helper T (TFH) cells and is expressed by the neoplastic T cells of some types of malignant T-cell lymphoma, including mycosis fungoides (MF) and Sézary syndrome (SS). Reported results of PD-1 staining in MF and SS are, however, conflicting. DESIGN Formalin-fixed, paraffin-embedded skin biopsy specimens were stained for PD-1. In addition, PD-1+ cases were stained with antibodies against BCL6, CXCL13, and CD10 to find possible relationship with TFH cells. SETTING Tertiary referral center for cutaneous lymphomas. PATIENTS Twenty-seven patients with SS and 60 patients with MF, including 8 patients with E-MF. RESULTS In patients with SS, expression of PD-1 by more than 50% of the neoplastic T cells was observed in 24 of 27 cases (89%). In contrast, PD-1 expression by more than 50% of neoplastic T cells was found in only 8 of 60 patients with MF (13%), including only 1 of 8 patients with E-MF (12%). In PD-1+ cases, serial skin sections showed that CXCL13 and BCL6 generally stained 25% to 50% of the PD-1+ cells, while expression of CD10 was uncommon. CONCLUSION The results of the present study show differential expression of PD-1 between SS and MF/E-MF, which provides further support for the view that SS and MF are distinct entities.