In this study we investigated whether programmed death-1 (PD-1) could serve as a useful diagnostic marker to differentiate between primary cutaneous CD4+ small/medium-sized pleomorphic T-cell lymphoma (PCSM-TCL) and cutaneous pseudo-T-cell lymphomas on the one hand and other types of cutaneous T-cell lymphomas (CTCLs) on the other. Formalin-fixed, paraffin-embedded skin biopsies from 26 patients with PCSM-TCL or pseudo-T-cell lymphoma, including 1 patient with a lymphomatoid drug eruption, and 52 skin biopsies from other types of CTCLs were stained for PD-1. In addition, PD-1-positive cases were stained with antibodies against BCL6, CXCL13, and CD10 to determine a possible relationship with follicular helper T (TFH) cells. In all 26 cases of PCSM-TCL or pseudo-T-cell lymphoma, the medium-sized to large-sized atypical T cells consistently expressed PD-1, BCL6, and CXCL13 but not CD10. PD-1 expression was found in only 2 of 21 cases of mycosis fungoides and in only 2 of 16 cases of cutaneous peripheral T-cell lymphoma, unspecified. All 4 patients with an aggressive epidermotropic cytotoxic CD8+ CTCL and all 11 cases with a primary cutaneous CD30+ lymphoproliferative disorder were negative for PD-1. In conclusion, PD-1 is typically expressed by atypical cells in PCSM-TCL and pseudo-T-cell lymphoma but is not expressed or is rarely expressed in other types of CTCLs. Therefore, it may serve as a suitable adjunct in differential diagnosis. Our results demonstrate that the atypical cells in PCSM-TCL and pseudo-T-cell lymphomas share a common TFH phenotype and support the view that most cases classified nowadays as PCSM-TCL are identical to cutaneous pseudo-T-cell lymphomas described previously.