OBJECTIVE The revised 2009 International Federation of Gynecology and Obstetrics (FIGO) staging system for endometrial cancer included many changes over the 1988 system, particularly for stage I subgroups. We sought to describe the overall survival (OS) of women with stage I endometrial cancer and examine how the estimated stage-specific OS is altered in the 2009 system. METHODS A prospectively maintained institutional endometrial database was analyzed. All patients underwent primary surgery between January 1993 and June 2009. RESULTS Data from 1658 women were analyzed, including 1307 patients with FIGO 1988 stage I disease. The 5-year OS for the 1988 stages IA (92.4%), IB (87.3%), and IC (75.7%) significantly differed (P < 0.001). When patients were restaged using the 2009 system, we identified 1411 stage I patients with 5-year OS for 2009 stage IA of 89.2%, versus OS of 75.1% for IB (P = 0.001). The adjusted concordance probabilities for the 1988 stage I group and 2009 stage I group were 0.612 (SD, 0.0014) and 0.536 (SD, 0.0111), respectively. CONCLUSIONS The 1988 FIGO classification of stage I endometrial cancer correctly identified 3 subgroups of patients who had significantly different OS. Specifically, 1988 FIGO stages IA and IB had distinct oncologic outcomes. The revised 2009 system eliminates the most favorable group from the new classification system, and estimates of stage-specific OS for stage IB are substantially altered by the changes made in 2009. The revised system for stage I did not improve its predictive ability over the 1988 system. These data highlight the importance of developing individualized risk-prediction models and nomograms in endometrial cancer.