Background. Nitric oxide (NO) is a free radical known to be a major regulator of vascular tonus, to inhibit cell proliferation, induce apoptosis, and be a mediator of macrophage cytostatic and cytotoxic effects. Recently, NO synthesis has been reported to be elevated in different cancers and is expected to promote metastasis by maintaining a vasodilator tone in blood vessels in and around the tumour. Two different common genetic polymorphisms were found on endothelial NO synthase (NOS3) gene: Glu298Asp on exon 7 and T-->786C in the promoter region.Purpose. To evaluate the impact of the NOS3 polymorphisms on vascular invasion and metastasis in breast cancer patients.Design. Two NOS3 gene polymorphisms (Glu298Asp and T-->786C) were genotyped in 71 patients operated for breast cancer and followed for 6-30 months (median 21). A control population of 91 age and sex matched tumour-free subjects was also genotyped for the same polymorphisms.Results. The distribution of both polymorphisms was not different between cases and controls. In patients without vascular invasion, T allele frequency was significantly lower than in patients with vascular invasion (p=0.033). At the end of the follow-up, T allele frequency was found to be less frequent in the metastasis free group than normal population (0.51 vs 0.64; p=0.047).Conclusion. Our results suggest that T allele reduction at the NOS3 promoter region may reduce vascular invasion in breast cancer and consequently reduce metastatic spread and be a favorable prognostic factor. These results need further validation in larger studies.