We have successfully treated over two hundred high-grade glioma (HGG) patients with immunotherapy consisting of vaccination with autologous dendritic cells (DCs) loaded with autologous tumour lysate. It has been documented that regulatory T cells (Treg) can counteract anti-tumour immune responses. Therefore, monitoring of Treg in these patients is essential. Up till now, Treg have been characterized based on the expression of the transcription factor Foxp3. Here, we validated IL-7 receptor alpha subunit (CD127)dim expression as a marker for human Treg within HGG patients, as a less laborious assay for routine use in tumour vaccination trials. We noted a strong positive correlation between Foxp3 expression and CD127dim expression in CD4+CD25+ and CD4+ cells. The suppressive function of CD4+CD127dim cells was assessed in an allogeneic mixed lymphocyte reaction (MLR). We conclude that CD127 staining is a fast, well-suited and reproducible Treg monitoring tool in HGG patients treated with immunotherapy.