Serum CA 125 levels were assayed from 44 normal healthy women, 153 patients with benign pelvic masses, and 58 patients with malignant pelvic masses. CA 125 levels were less than 35 U/mL in 42 of the 44 normal women and were greater than 35 but less than 65 U/mL in the other two women. Among 153 patients with benign pelvic masses, CA 125 levels greater than 35, 65, or 194 U/mL were detected in 61 (39.9%), 31 (20.3%), and eight (5.2%) patients, respectively. Of 58 patients with malignant pelvic masses, CA 125 results were greater than 35, 65, or 194 U/mL in 48 (82.8%), 45 (77.6%), and 38 (65.5%), respectively. Among the latter group, the positivity rates of 30 patients with epithelial ovarian cancers were 100, 93, and 80%, respectively. This study suggests that defining positive serum CA 125 levels as those greater than 35 U/mL is of limited clinical value because there is a 39.9% false-positive rate in patients with benign disease. However, serum CA 125 values greater than 65 U/mL may be considered positive in clinically normal women. Serum CA 125 greater than 194 U/mL, representing the units at the 95th percentile for 153 patients with benign pelvic masses, is defined as a new positivity criterion, and could be used to differentiate malignant tumors from benign pelvic masses.