The aim of this study was to investigate the effects of IL‐17‐producing T cells, including Th17 and Tc17 cells, on acute graft‐versus‐host disease (aGVHD) in patients who had undergone granulocyte colony‐stimulating factor (G‐CSF)‐mobilised peripheral blood progenitor cell (PBPC) and G‐CSF‐primed bone marrow (G‐BM) transplantation. Allografts from forty‐one patients were analysed for IL‐17‐producing T cells with respect to aGVHD. Furthermore, ten patients with aGVHD onset were monitored for the presence of Th17 cells in the peripheral blood by flow cytometry. Patients who received a higher dose of Th17 cells in the G‐BM (>8.5×10⁴/kg, p=0.005) or a higher dose of Tc17 cells in PBPC (>16.8×10⁴/kg, p=0.001) exhibited a higher incidence of aGVHD. An increased Th17 population (up to 4.99% CD4⁺ T lymphocytes) was observed in patients with aGVHD onset. In contrast, the percentage of Th17 population decreased drastically in aGVHD patients following treatment to achieve partial and complete remission (p=0.013 and p=0.008, respectively). All percentages of Th17 and Tc17 cells were significantly reduced after in vivo G‐CSF application. Our results suggested that IL‐17‐producing T cells contributed to aGVHD. The application of G‐CSF in vivo aided in reducing the occurrence of aGVHD through a decrease in IL‐17 secretion by T cells.