Purpose: Malignant gliomas are the most common tumors in the central nervous system with a poor prognosis. Recently, CD4⁺ cytotoxic T cells (CTLs) are being increasingly recognized as possessing antitumor capacity. However, their presence, activity and regulation in glioma have not been investigated in detail. Methods: To examine this, 72 grade II and grade III Han Chinese glioma patients and 30 Han Chinese healthy controls were investigated. Results: We found that compared to healthy controls, glioma patients had significantly upregulated frequencies of circulating CD4⁺ CTLs, identified by the expression of granzyme A (GzmA), granzyme B (GzmB) and/or perforin. The stimulated CD4⁺ CTLs in grade II and grade III glioma patients also had less proliferative ability than those in healthy controls, a feature of suppression that progressed with tumor grade. The frequencies of GzmB-expressing circulating CD4⁺ CTLs were directly associated with prognosis. We hypothesized that the programed death 1 (PD-1)/PD-ligand 1 (L1) interaction possibly contributed to the suppression of CD4⁺ CTLs in grade II and grade III glioma, since an upregulation of PD-1 was observed on CD4⁺ CTLs in glioma compared to those in the healthy individuals. Blockade of the PD-1/PD-L1 interaction with neutralizing antibodies significantly increased the proliferation and granzyme or perforin production by CD4⁺ CTLs in grade II and grade III glioma patients. Conclusions: These data suggest that the CD4⁺ CTLs in grade II and grade III glioma patients contribute to antitumor immunity and could be suppressed by PD-1 signal transduction.