Colorectal cancer is a significant worldwide health problem, and an altered immunoresponse plays an important role in colorectal tumorigenesis and cancer progression. T helper 17 (Th17) lymphocytes (a subgroup of CD4(+) T cells), together with the related cytokines, such as interleukin (IL)-6 and IL-17A, participate in cancer-related immunity. In this study, we measured the percentage of Th17 cells in peripheral blood mononuclear cells (PBMCs) and the levels of IL-17A and IL-6 in serum samples or tumor tissues, prepared from 40 colorectal cancer patients (a median age of 75 years), 32 age-matched healthy controls (a median age of 74 years), and 30 young healthy controls (a median age of 26 years). The percentage of Th17 cells in PBMCs and the serum IL-6 levels were higher in cancer patients than those in elderly controls, and they were associated with tumor progression. Moreover, the percentage of Th17 cells and the serum IL-6 levels were higher in elderly healthy controls than those in young healthy controls. In contrast, serum IL-17A levels were similar in the cancer patients and the elderly controls, although its serum levels were higher than those in young controls. Importantly, Th17 cells were accumulated in the intratumor and peritumor regions. In conclusion, the percentage of Th17 cells and the serum IL-6 level are significantly increased with aging, and their higher levels are correlated with colorectal cancer progression. The percentage of Th17 cells in PBMCs and the serum IL-6 level are potential biomarkers for predicting disease progression.