Th17 cells and CD4(+)CD25(+) regulatory T (Treg) cells have been reported to share reciprocal developmental pathways but exhibit opposite effects, and the balance between them controls inflammation and autoimmune diseases. However, information regarding Th17/Treg cells in cancer-bearing hosts is still limited. In the present study, we investigated the distribution of Th17 cells in relation to Treg cells in gastric cancer patients, and evaluated how the imbalance in Th17/Treg cells in gastric cancer correlates with clinical and pathological parameters. We observed that the accumulation of Th17 and Treg cells in the tumor microenvironment was gradually increased according to disease progression, leading to an imbalance in Th17/Treg cells in gastric cancer patients. TGF-beta and interleukin (IL)-6 present in the gastric cancer microenvironment promoted the differentiation and expansion of Th17 cells, and increased numbers of Th17 cells promoted tumor progression through promotion of inflammation by secretion of IL-17. Treg cells promoted tumor progression by helping cancer cells escape from host immunosurveillance by secreting TGF-beta, and a high level of TGF-beta in the tumor microenvironment promoted differentiation and expansion of Treg cells. In conclusion, the imbalance in Th17/Treg cells was involved in the development and progression of gastric cancer. A better understanding of the nature, regulation, and function of Th17 and Treg cells in tumor immunity may aid in the development of novel and effective immunotherapy for gastric cancer.