Serum CA 125 was assayed preoperatively in 40 patients with ovarian malignancies and in 44 normal individuals. The correlation between CA 125 and tumor histology, tumor burden, ascites, and the clinical stage was analyzed. CA 125 levels greater than 65 U/ml were defined as positive. The results revealed normal values in each of the 44 normal individuals. CA 125 was elevated in 35 of 40 (87.5%) patients with malignant ovarian tumors including 23 of 24 (95.8%) patients with epithelial ovarian cancers, 7 of whom had stage I disease. Serum CA 125 was also elevated in 12 of 16 (75%) patients with nonepithelial ovarian malignancies. The results suggest that the CA 125 level can be correlated with tumor histology, tumor burden, and ascites but not with clinical stage.