Obesity and overweight have become major medical and social problems. Both are increasing worldwide; two-thirds of the population in developed countries is obese or overweight. Obesity has been associated with many comorbidities, including diabetes and heart disease. Studies have also found that obesity is one of the risk factors involved in increased cancer incidence. Many obesity-related factors are responsible, including increased blood levels of insulin/IGF, IL-6, TNF-alpha, and leptin, and decreased blood levels of adiponectin. Recently, it has been shown that IL-17 levels increase in obese patients. IL-17 is well known to increase carcinogenesis; thus, increased IL-17 levels in obesity may contribute to increased cancer incidence in obesity. IL-17 could activate Src/PI3K, MAPK, Stat3, and PKC, resulting in carcinogenesis. It may also change the microenvironment of tumors. Thus, inhibition of IL-17 may have preventive and therapeutic implications in obese patients.