In the tumor microenvironment, interleukin (IL)-10 production has a pleiotropic ability to positively and negatively influence the function of innate and adaptive immunity against cancer. This study investigated whether IL-10 genetic polymorphisms that influence gene expression levels play a role in the risk and clinical course of clear-cell renal cell carcinoma (RCC). We analyzed the allelic and haplotype frequency formed by alleles at -1082(G/A), -819(C/T), and -592(C/A) of the IL-10 gene in RCC (n = 126) and healthy individuals (n = 176). The frequency of IL-10 polymorphic variants was similar between patients and controls. However, -1082 G/A IL-10 genotype showed a significant association with three prognostic indicators: advanced disease stage (p = 0.002), higher tumor size (p = 0.001), and presence of adenopathy (p = 0.006). Our results can be explained by the contradictory antitumor or pro-tumorigenic relationship between this molecule and cancer. Genotypes associated with high or low levels of IL-10 gene expression (GG or AA-1082 IL-10) were both associated with a more favorable course of the disease. We propose the hypothesis that the -1082 GA medium expression genotype confers a tumor-promoting phenotype, likely resulting from the immunosuppressive effects of anti-tumor Th-1 responses in conjunction with the insufficient inhibition of tumor angiogenesis at this intermediate level of IL-10 expression.