Uridine diphosphate-glucuronosyltransferases (UGTs) are a family of phase II-metabolizing enzymes involved in glucuronic acid conjugation of sex steroid hormones. UGT1A1 and UGT2B7 are expressed in the uterus and involved in the conjugation and elimination of estrogens. Chronic exposure to estrogens is associated with endometrial cancer. Functional polymorphisms have been identified in UGT1A1 and UGT2B7. We hypothesized that these variants may be associated with endometrial cancer risk. We conducted a case-control study nested within the Nurses' Health Study and the Women's Health Study to investigate the associations between five polymorphisms and endometrial cancer risk using 593 invasive endometrial cancer cases and 1545 controls. We did observe the suggestion of an inverse association with homozygote variant carriers of UGT1A1*28 and endometrial cancer risk. We did not observe significant associations between individual single nucleotide polymorphisms and UGT1A1 haplotypes and endometrial cancer risk. Our data suggest that these UGT polymorphisms do not contribute significantly to endometrial cancer risk.