Background: The prognostic relevance of uncommon epithelial ovarian cancer (EOC) histological subtypes remains controversial. The Gynecologic Cancer InterGroup (GCIG) initiated this meta-analysis to assess the relative prognosis of women with a diagnosis of rare EOC histologies from completed, prospectively randomized studies performed by cooperative GCIG study groups. Methods: Studies eligible for analysis included first-line treatment of at least 150 patients with stage III/IV EOC treated with a platinum/taxane-based regimen. Collaborating groups were to provide patient-level data. Serous acted as the reference histology, and a proportional hazards model was used to estimate the relative rate of progression or death. Results: Data on 8704 women with stage III/IV EOC from 7 randomized trials were included in these analyses. Two hundred twenty-one patients (2.5%) had clear cell carcinoma; 264 (3.0%), mucinous; and 36 (0.4%), transitional cell. The mean age of patients with serous histology was greater than those with mucinous (4.1 years) and clear cell (2.6 years, P <0.001). Mucinous, clear cell, and transitional cell tumors were more likely to be completely resected than serous (P <0.05). When controlling for age and residual disease, mucinous and clear cell tumors had shorter times to progression (hazards ratio [HR], 2.1; 95% confidence interval [CI], 1.8-2.4 and HR, 1.6; 95% CI, 1.4-1.9, respectively) and death (HR, 2.7; 95% CI, 2.3-3.1 and HR, 2.2; 95% CI, 1.8-2.6, respectively) compared with serous. The median overall survival for serous, clear cell, mucinous, and endometrioid histologies were 40.8, 21.3, 14.6, and 50.9 months. Conclusions: Mucinous and clear cell carcinomas are independent predictors of poor prognosis in stage III/IV EOC. Studies targeting these rare histological subtypes are warranted and will require significant intergroup collaboration.