Purpose: The prognostic significance of p53 mutations and overexpression in advanced epithelial ovarian cancers was examined in primary tumors from 125 patients participating in a Gynecologic Oncology Group randomized phase III treatment protocol. Patients and Methods: Mutational analysis of p53 was performed in RNA or genomic DNA extracted from frozen tumor. An immunohistochemistry assay was used to detect p53 overexpression in fixed tumor. Results: There were 81 patients (74%) with a single mutation, three patients (3%) with two mutations, and 25 patients (23%) lacking a mutation in exons 2 to 11 of p53. Although most mutations occurred within exons 5 to 8, mutations outside this region were observed in 11% of patients. A mutation in exons 2 to 11 of p53 was associated with a short-term improvement in overall survival and progression-free survival. Adjusted Cox modeling demonstrated a 70% reduction in risk of death (P = .014) and a 60% reduction in risk of disease progression (P = .014) for women with such mutations. However, these striking risk reductions increased over time (P < .02) and eventually disappeared with longer follow-up. Over-expression of p53 was observed in 55 patients (100%) with only missense mutation(s), seven patients (32%) with truncation mutations, and eight patients (40%) lacking a mutation in exons 2 to 11. Over-expression of p53 was associated with tumor grade but not with patient outcome. Conclusion: Alterations in p53 are a common event in advanced epithelial ovarian cancer. A mutation in p53, but not overexpression of p53, is associated with a short-term survival benefit. Additional studies are required to define the roles that p53 plays in regulating therapeutic responsiveness and patient outcome.