ABO blood type has been associated with risk of several malignancies. However, results are not consistent. In this population-based case-control study including 1204 incident endometrial cancer cases and 1212 population controls, we examined the association of self-reported serologic blood type with endometrial cancer risk using a logistic regression model. Women with endometrial cancer were more likely to have blood type A. Compared to women with blood type O, the adjusted odds ratios for endometrial cancer were 1.00 [95% confidence interval (CI), 0.79-1.28] for type B, 1.24 (95% CI, 0.90-1.69) for type AB, and 1.50 (95% CI, 1.19-1.90) for type A. A significant dose-response relationship was observed for cancer risk and level of antigen A (P for trend = 0.0003). The positive association of blood type A with cancer risk was observed regardless of menopausal status, body mass index, oral contraceptive use, or family cancer history. Our results suggest that ABO blood type may be involved in the development of endometrial cancer.