Background/Aim: Interleukin-16 (IL-16) is reported to play an important role in inflammation, carcinogenesis and tumoricidal processes, however, the contribution of IL-16 genotype to oral carcinogenesis is still largely unrevealed. Thus, the study aimed to investigate the contribution of IL-16 genotypes to Taiwan oral cancer risk. Materials and Methods: The genotypes of IL-16 rs4778889, rs11556218, and rs4072111 were revealed among 958 oral cancer cases and 958 control subjects by polymerase chain reaction-based restriction fragment length polymorphism (PCR-RFLP). Results: First, the distributions of genotypic (p=0.0004) and allelic (p=0.0001) frequencies of IL-16 rs11556218 were significantly different between the case and control groups. In detail, the frequencies of IL-16 rs11556218 TG and GG were 28.1 and 5.8%, respectively, among oral cancer patients, significantly higher compared to those among controls (25.0% and 2.7%, respectively). Second, no difference was observed regarding IL-16 rs4778889 or IL-16 rs4072111. Last, there was a synergistic effect of betel quid chewing behavior and risky IL-16 rs11556218 genotype on oral cancer risk. Conclusion: The study indicates that the IL-16 rs11556218 G allele synergistically interacts with betel quid chewing behavior, contributing to increased risk of oral cancer in Taiwanese. From the viewpoint of epidemiology, oral cancer is the tenth most commonly diagnosed cancer worldwide, with the highest incidence density in Taiwan (1). According to the updated annual report from the Taiwan government, oral cancer is the fourth cause of cancer-related deaths among males in Taiwan and the fifth among all Taiwanese (2). For many years, betel quid chewing habit has been identified as the most effective environmental contributor to oral cancer risk for Taiwanese (3). Although the surgery, therapy and medical caring services for oral cancer have made rapid progress during the past decade, the prevalence and death rate of oral cancer are still very high in Taiwan. Thus, early detection and prediction biomarkers for oral cancer risk are in urgent need. Interleukin-16 (IL-16) is encoded by the IL-16 gene located on chromosome 15q26.3, composed of 631 amino acids and cleaved by caspase 3 to the active IL-16 protein, comprising the 121 amino acid C-terminal (4-6). IL-16 is in charge of activating several types of cells, including CD4+ T cells, monocytes, macrophages, eosinophils, and dendritic cells, and promotes the secretion of inflammatory cytokines, such as IL-1 (7), TNF-, and IL-15 (8). Interestingly, a 1759 This article is freely accessible online.