Interleukin (IL)-6 is known to be involved in the pathogenesis of ovarian cancer. We investigated a common G/C polymorphism at position -174 of the IL-6 gene (IL6) promoter in 121 patients with ovarian cancer using pyrosequencing. Presence of at least one mutant allele was associated with early tumor stage as well as an expanded length of disease-free (DFS) and overall survival with a dose-dependent effect regarding the carriage of 0, 1, and 2 alleles. In a multivariate Cox regression model incorporating tumor stage and residual tumor mass, presence of the -174 C allele of IL6 was the best predictor of DFS. We conclude that the mutant -174 C allele of IL6 influences the biological phenotype of ovarian cancer because it is associated with early stage and improved DFS and overall survival.