Breast cancer is among the most common causes of cancer-related death in women worldwide. Previous studies have demonstrated an association between prolonged estrogen exposure and increased risk of breast cancer. Uridine 5'-diphospho-glucuronosyltransferase 1-1 (UGT1A1) plays a significant role in the detoxification of estrogens. Two major genetic polymorphisms have been identified in the UGT1A1 locus. UGT1A1*28 has been previously linked to increased risk of breast cancer. The aim of this study was to elucidate the possible correlation between UGT1A1*6, a single nucleotide polymorphism causing a Gly71Arg substitution, and breast cancer susceptibility. Forty-six women diagnosed with breast cancer, 15 patients with gastrointestinal cancer, and 13 healthy women were recruited to this study. The genotype in the polymorphic UGT1A1 locus was determined by DNA sequencing. The frequency of each genotype was compared among the three groups. The frequency of the UGT1A1*6 allele was significantly higher in breast cancer and gastrointestinal cancer patients than that in healthy females (both P < 0.05). No significant associations were observed between the UGT1A1*6 polymorphism and estrogen receptor, progesterone receptor, HER-2 expression status, menstrual status, or metastasis (all P > 0.05). Therefore, the UGT1A1*6 polymorphism was deduced to be a risk factor for breast cancer in women of Han Chinese ethnicity. UGT1A1 may serve as a therapeutic target for the prevention and treatment of breast cancer and other estrogen-related diseases.