The p53 suppressor gene protein expression was studied with immunoperoxidase staining in 61 serous tumors of the ovary. Twenty four (53%) of the 45 histologically malignant tumors were positive for p53, whereas neither the six benign nor the 10 tumors of borderline malignancy showed positive staining, except for one borderline tumor with atypia and increased mitotic activity. Therefore, p53 immunostaining may have diagnostic value in discriminating between borderline and malignant serous ovarian tumors. Malignant ovarian tumors with negative staining for p53 were associated with a 67% 3-year crude survival rate as compared with only an 18% 3-year survival rate if p53 staining was positive (p = 0.002). In a multivariate analysis, the p53 staining was the most important prognostic factor, with a relative risk of 4.2 (95% confidence interval, 1.8-9.9) followed by the FIGO stage (2.1, 1.3-3.5). We conclude that immunohistochemical p53 suppressor gene protein expression analysis has both diagnostic and prognostic value.