The prognostic significance of the serum CA 125 level was studied in 687 patients with invasive epithelial ovarian malignancies. The samples were collected preoperatively in 200 and postoperatively in 487 patients. Median follow-up was 27 months (range 3-84). The serum CA 125 level was elevated preoperatively in 90% of cases, with a median value of 429 U/mL. In patients with evidence of disease at the time of sampling, the CA 125 serum level correlated directly to tumor stage, tumor load, and histologic grade. Using Cox multivariate analysis, the preoperative serum CA 125 level had no independent prognostic significance, whereas the postoperative level did. In patients without residual disease after primary surgery, histologic type (P less than .0001), postoperative CA 125 level with 35 U/mL as the cutoff value (P = .0009), and tumor grade (P = .034) were independent prognostic factors for survival. For those with residual tumor after primary surgery, histologic type (P less than .0001), postoperative treatment (P = .0002), size of residual disease (P = .0005), and postoperative serum CA 125 level with 65 U/mL as a cutoff (P = .003) were independent prognostic factors.