The 825C>T polymorphism in the gene for the G-protein beta3 subunit (GNB3) has been linked to the occurrence of a splice variant of GNB3 and distinct cellular and metabolic features and may be associated with malignant disease. 500 patients with histologically confirmed breast cancer and 500 female age-matched healthy control subjects were genotyped for the GNB3 polymorphism to analyze its role for breast cancer. Prevalences of GNB3 CC, CT and TT genotypes were similar among patients (49.7, 39.8, 10.5%) and controls (50.1, 42.4, 7.5%, P = 0.25). The GNB3 genotype was furthermore not linked to tumor size, histological grading, estrogen or progesterone receptor status and age at diagnosis. In an exploratory analysis, carriage of a 825-T allele was associated with a longer metastasis-free period in patients with primary low-grade breast cancer, but not in those with primary high-grade breast cancer (Cox regression, P = 0.025). We conclude that the GNB3 825C>T polymorphism does not appear to be associated with breast cancer risk, but may influence development of metastasis in low-grade tumors.