Estrogen receptor (ER) is a ligand-activated transcription factor that mediates estrogen actions in target tissues. Several common polymorphisms of the ER-alpha gene have been reported to be associated with alterations in receptor expression and function. We evaluated the hypothesis that genetic polymorphisms in the ER-alpha gene may be associated with breast cancer risk in a population-based case-control study conducted in urban Shanghai during 1996-1998. Two RFLPs at the ER-alpha gene locus, denoted as PvuII and XbaI, were examined in 1069 breast cancer cases and 1166 age frequency-matched controls. PvuII polymorphism was associated with an increased risk of breast cancer with the age-adjusted odds ratios for genotypes Pp and pp being 1.3 [95% confidence interval (CI), 1.0-1.7) and 1.4 (95% CI, 1.1-1.8), respectively, comparing to genotype PP. The XbaI polymorphism was associated with a nonsignificantly elevated risk. The odds ratios for genotypes Xx and xx were 1.2 (95% CI, 0.7-1.9) and 1.3 (95% CI, 0.8-2.0), respectively, and the elevated risks were mainly confined to older or postmenopausal women. No apparent synergetic effect of these two polymorphisms was identified. Results of this study indicate that genetic polymorphisms in the ER-alpha gene may play a role in the etiology of breast cancer.