Objective: We propose a new staging system for stage I endometrial cancer and compare its performance to the 1988 and 2009 International Federation of Gynecology and Obstetrics (FIGO) systems. Methods: We analyzed patients with 1988 FIGO stage I endometrial cancer from January 1993 to August 2011. Low-grade carcinoma consisted of endometrioid grade 1 to grade 2 lesions. High-grade carcinoma consisted of endometrioid grade 3 or nonendometrioid carcinomas (serous, clear cell, and carcinosarcoma). The proposed system is as follows: IA. Low-grade carcinoma with less than half myometrial invasion. IA1 Negative nodes IA2 No nodes removed. IB. High-grade carcinoma with no myometrial invasion. IB1 Negative nodes. IB2 No nodes removed. IC. Low-grade carcinoma with half or greater myometrial invasion. IC1 Negative nodes. IC2 No nodes removed. ID. High-grade carcinoma with any myometrial invasion. ID1 Negative nodes. ID2 No nodes removed. Results: Data from 1843 patients were analyzed. When patients were restaged with our proposed system, the 5-year overall survival significantly differed (P < 0.001): IA1, 96.7%; IA2, 92.2%; IB1, 92.2%; IB2, 76.4%; IC1, 83.9%; IC2, 78.6%; ID1, 81.1%; and ID2, 68.8%. The bootstrap-corrected concordance probability estimate for the proposed system was 0.627 (95% confidence interval, 0.590-0.664) and was superior to the concordance probability estimate of 0.530 (95% confidence interval, 0.516-0.544) for the 2009 FIGO system. Conclusions: By incorporating histological subtype, grade, myometrial invasion, and whether lymph nodes were removed, our proposed system for stage I endometrial cancer has a superior predictive ability over the 2009 FIGO staging system and provides a novel binary grading system (low-grade including endometrioid grade 1-2 lesions; high-grade carcinoma consisting of endometrioid grade 3 carcinomas and nonendometrioid carcinomas).