BACKGROUND: A study was undertaken to use the 2-tier system to reclassify the grade of serous ovarian tumors previously classified using the International Federation of Gynecology and Obstetrics (FIGO) 3-tier system and determine the progression-free survival (PFS) and overall survival (OS) of patients treated on Gynecologic Oncology Group (GOG) Protocol 158. METHODS: The authors retrospectively reviewed demographic, pathologic, and survival data of 290 patients with stage III serous ovarian carcinoma treated with surgery and chemotherapy on GOG Protocol 158, a cooperative multicenter group trial. A blinded pathology review was performed by a panel of 6 gynecologic pathologists to verify histology and regrade tumors using the 2-tier system. The association of tumor grade with PFS and OS was assessed. RESULTS: Of 241 cases, both systems demonstrated substantial agreement when combining FIGO grades 2 and 3 (overall agreement, 95%; kappa statistic, 0.68). By using the 2-tier system, patients with low-grade versus high-grade tumors had significantly longer PFS (45.0 vs 19.8 months, respectively; P=.01). By using FIGO criteria, median PFS for patients with grade 1, 2, and 3 tumors was 37.5, 19.8, and 20.1 months, respectively (P=.07). There was no difference in clinical outcome in patients with grade 2 or 3 tumors in multivariate analysis. Woman with high-grade versus low-grade tumors demonstrated significantly higher risk of death (hazard ratio, 2.43; 95% confidence interval, 1.17-5.04; P=.02). CONCLUSIONS: Women with high-grade versus low-grade serous carcinoma of the ovary are 2 distinct patient populations. Adoption of the 2-tier grading system provides a simple yet precise framework for predicting clinical outcomes. © 2011 American Cancer Society.