Serum and tissue levels of interleukin-6 (IL-6) have been implicated in the biological phenotype of breast carcinoma. A common G/C polymorphism at position -174 of the IL-6 promoter can influence the expression level of this gene. We therefore investigated for associations between this polymorphism and various phenotypic features in a series of 256 breast cancers. Individuals who were homozygous for the C allele (n = 55) were more likely to have higher-grade tumours (P = 0.039) with ductal histology (P = 0.030) compared to those harbouring at least one wild-type G allele (n = 201). Homozygosity for the C allele was also associated with significantly worse overall survival (P = 0.031). We conclude that the -174 C allele of IL-6 is associated with a more aggressive breast cancer phenotype.