Interleukin-17A-producing T cells (IL-17A(+)T) (IL-17A(+)CD4(+) Th17, IL-17A(+)CD8(+) Tc17 and IL-17A(+) gamma delta T17 cells) and associated cytokines (IL-17A, IL-23 and IL-1 beta) play crucial roles in inflammation-associated diseases, such as infection, autoimmunity and tumors. Th17 cells promote human gastric cancer (GC), although the source of intracellular IL-17A and the roles of Tc17 and gamma delta T17 cells remain poorly understood. In this study, the frequencies of circulating Th17 and gamma delta T17 cells in patients with GC were found to be significantly increased compared to those in healthy donors; however, Tc17 cells were decreased in these patients, and a negative relationship was found between the frequencies of Th17 and Tc17 cells. Moreover, the cytokine IL-17A was found to be produced mainly by Th17 cells in human peripheral blood. Similarly, serum cytokine levels and relative mRNA expression levels of IL-17A, IL-23 and IL-1 beta were significantly increased in patients with GC, and the frequency of Th17 cells was closely associated with serum IL-17A concentrations in patients with GC. Additionally, Th17 cells and associated cytokines were present at significantly different levels during the progression and metastasis of GC, as were Tc17 and gamma delta T17 cells. Taken together, these findings suggest that IL-17A(+) T cells and associated cytokines might play crucial roles in human GC progression and metastasis and thus represent potential targets for treatment.