Extra-ovarian sex hormone production plays an important role in endometrial cancer in postmenopausal women. Aromatase, which is encoded by CYP19A1, is a key enzyme in estrogen biosynthesis after menopause. To examine the association between polymorphisms in CYP19A1 and endometrial cancer risk among postmenopausal Japanese women, we conducted a hospital-based case control study in 48 patients with histologically diagnosed incident endometrial cancer and 253 non-cancer control subjects. Information on lifestyle factors was obtained from a self-administered questionnaire. Twenty-five tag SNPs (single nucleotide polymorphisms) of CYP19A1 were examined by TaqMan methods and haplotype blocks were identified by LD analysis. Associations were assessed by an unconditional logistic regression model adjusted for potential confounders. We found no significant association between CYP19A1 genotypes and haplotypes and endometrial cancer risk. However, among women with a BMI (body mass index) >23, significantly positive associations were observed for rs2899473, rs1865803, rs16964220, rs2008691, rs17647707, rs17647719, rs1902586, rs936306, and rs1004982, while negative associations were seen for rs1902585, rs752760 and rs2445768. These showed significant interactions with BMI. Further, of the six haplotype blocks identified, the haplotype CTT of block 1, GATA of block 5 and CA of block 6 showed statistically significant interactions with BMI. These results suggest that CYP19A1 polymorphisms might play an important role in the etiology of endometrial cancer, and that the effect of these polymorphisms might be influenced by BMI.