Obesity has been shown to increase the risk or be associated with numerous conditions from cardiovascular disease and type II diabetes to erectile dysfunction and osteoarthritis. Obesity may also be associated with numerous cancers, and kidney cancer or renal-cell cancer (RCC) may have one of the strongest correlations to obesity compared with cancer at any other site. Almost every epidemiologic investigation has demonstrated an association that tends to affect women more than men, but both genders are impacted. In general, past studies suggest that with increasing weight, a threshold point exists whereby a certain range of body mass index dramatically changes risk. Men and women at the most extreme ends of obesity tend to have the highest risk or only risk in past studies. Individuals at the more extreme ends of obesity may be affected by an almost indefinite number of mechanisms and exposures that could determine incidence and possibly prognosis. For example, higher estrogen levels, elevated insulin levels, a greater concentration of growth factors in adipose tissue, hypertension, cholesterol metabolism abnormalities, and immune malfunction are just some of the potential mechanisms that may increase kidney cancer risk. Obese individuals may also have lower serum levels of vitamin D and engage in less physical activity. Smoking or genetic predisposition to RCC may synergistically contribute to the effect of obesity on risk. The potential mechanisms and associations are numerous and complex. Regardless of the actual cancer risk now and in the future, the overall effect of obesity on general health is clear, and this should be kept in mind in the discussion between health professional and patient.