Breast cancer is the most frequent type of cancer diagnosed among women worldwide and also in Thailand. Estrogen and estrogen receptors exert important roles in its genesis and progression. Several cytokines have been reported to be involved in the microenvironment that promotes distant metastasis via modulation of immune and inflammatory responses to tumor cells. Estrogen receptor genetic polymorphisms and several cytokines have been reported to be associated with breast cancer susceptibility and aggressiveness. To investigate roles of genetic polymorphisms in estrogen receptor alpha (ESR1) and interleukin 6 (IL6), breast cancer patients and control subjects were recruited from the Division of Head, Neck and Breast Surgery (Siriraj Hospital, Bangkok, Thailand). Polymorphisms in ESR1 (rs3798577) and IL6 (rs1800795 and rs1800797) were evaluated by real-time PCR in 391 breast cancer patients and 79 healthy controls. Associations between genetic polymorphisms and clinicopathological data were determined. There was no association between genetic polymorphisms and breast cancer susceptibility. However the ESR1 rs3798577 CT genotype was associated with presence of lymphovascular invasion (OR=2.07, 95%CI 1.20-3.56, p=0.009) when compared to the TT genotype. IL6 rs1800795 CC genotype was associated with presence of extranodal extension (OR= 2.30, 95%CI 1.23-4.31, p=0.009) when compared to the GG genotype. Survival analysis showed that IL6 rs1800797 AG or AA genotypes were associated with lower disease-free survival. These findings indicate that polymorphisms in ESR1 and IL6 contribute to aggressiveness of breast cancer and may be used to identify high risk patients.