The purpose of this study was to investigate the frequency of p53 overexpression in the primary ovarian tumors of patients with stages II and III serous borderline tumors (SBTs) and to determine the relationship between p53 overexpression and risk of progression/recurrence and survival. Of 112 patients with stages II-IV SBTs, paraffin-embedded tissue from the primary ovarian tumor was available in 68 cases. Immunohistochemical staining for p53 was performed. Clinical information was abstracted from the medical records. The major end points selected for analysis were time to progression/relapse, disease-free survival, overall survival, and cause-specific survival. Univariate and multivariate regression analyses were also performed. The median patient age was 37 years (range, 17-67 years). Twenty-two patients had stage II disease, and 46 had stage III disease. The mean follow-up time was 105 months. Nineteen patients (28%) had either disease progression (1 patient) or relapse (18 patients). Eleven patients died: 10 patients died of their tumor, and 1 patient died of other causes. Thirteen cases (19%) had positive immunostaining for p53. Overexpression of p53 was significantly associated with an increased probability of progression/recurrence (P = 0.005) and a decreased overall survival (P = 0.012). After adjusting for age, International Federation of Gynecology and Obstetrics (FIGO) stage, the presence of residual tumor, and the presence of invasive implants, patients whose tumors overexpressed p53 had a 4-fold increased risk of progression/ recurrence. Similarly, women whose tumor overexpressed p53 had an approximately 6-fold increased risk of death. p53 overexpression in the ovarian tumors of patients with stage II and III SBTs is significantly associated with increased probability of relapse and decreased overall survival. This information should provide better prognostic data to patients and their families and allow us to select patients who might benefit from postoperative treatment.