Previous studies of CA-125 levels from screening trials for ovarian cancer have indicated that serial CA-125 levels may identify cases better than a fixed CA-125 cutoff. We conducted a study to assess the screening performance of the risk of ovarian cancer calculation based on serial CA-125 levels from prospectively collected serum samples compared with a fixed CA-125 cutoff. The calculation was applied to data from a prospective trial of screening for ovarian cancer involving 22,000 postmenopausal women older than 45 years. The analysis was performed using 33,621 CA-125 results from 9,233 women for whom two or more serial samples were available. All serum samples from the patients with ovarian cancer were obtained before clinical detection. Sensitivity and specificity levels for preclinical detection of index cancers were calculated for various cutoffs for the risk and a single CA-125 measurement, and receiver operator curves were constructed. The risk calculation significantly improved the area under the curve from 84% to 93% compared with a fixed cutoff for CA-125 (P =.01). For a target specificity of 98%, the risk achieved a sensitivity of 86% for preclinical detection of ovarian cancer, whereas CA-125 achieved a sensitivity of 62%. The estimates of performance are unbiased, because the risk calculation was derived independent of the data from this trial. These results provide the first evidence that preclinical detection of ovarian cancer using serial CA-125 levels interpreted with the risk calculation significantly improves screening performance compared with a fixed cutoff for CA-125. The results justify the incorporation of the risk calculation in a prospective, randomized, controlled trial.