The purpose of the present study was to evaluate the information preoperative measurements of cancer antigen 125 (CA 125) may give concerning the nature of an ovarian tumor. Serum samples from 61 patients were analyzed and the results compared with regard to malignancy, FIGO-stage, and histologic type of the tumor. All patients were deemed to have a malignant tumor by clinical examination. Elevated serum levels (greater than 35 U/ml) were found in 44 patients and of these patients 38 had carcinomas, 5 had borderline tumors, and one had a benign tumor (false positive). Normal serum levels (less than or equal to 35 U/ml) were registered in 17 patients. Fourteen of these had benign tumors and 3 had malignant (false negative). The false positive sample and the 3 false negative samples observed in the present study result in a positive and negative predictive value of the CA 125 analysis of 98% (43/44) and 82% (14/17) respectively. The positive predictive value of the clinical examination was 75% (46/61). It is concluded that measurement of CA 125 is a valuable adjunct to the preoperative examination of patients with ovarian tumors.