Interleukin (IL)-16, a multifunctional cytokine, plays a fundamental role in inflammatory diseases, as well as in the development and progression of tumors. Genetic variation in the DNA sequence of the IL-16 gene may lead to altered cytokine production and/or activity, and this variation may modulate an individual's susceptibility to both colorectal cancer (CRC) and gastric cancer (GC). To test this hypothesis, we investigated the association of IL-16 gene polymorphisms with serum levels of IL-16 and the risk of CRC and GC in a Chinese population. We analyzed single-nucleotide polymorphisms of the IL-16 gene in 596 cancer patients (376 patients with CRC and 220 patients with GC), and also in 480 age- and sex-matched controls using polymerase chain reaction–restriction fragment length polymorphism and DNA sequencing methods. Serum IL-16 levels were measured by enzyme-linked immunosorbent assay. The rs11556218 T/G polymorphism of the IL-16 gene was significantly associated with the susceptibility to CRC and GC patients. Both male and female patients carrying the G allele had a significantly higher risk for developing CRC and GC compared with individuals carrying the T allele. Alternatively, women carrying the T allele (rs4072111 C/T) showed a decreased risk for CRC and GC compared with individuals carrying the C allele. In patients with CRC or GC, IL-16 serum levels were significantly higher than those in the healthy controls, although no significant association between IL-16 polymorphisms and serum levels of IL-16 was observed. Our data indicate that IL-16 polymorphisms may contribute to CRC and GC susceptibility.