Preoperative serum CA-125 levels were evaluated in 38 patients who underwent primary surgery for epithelial ovarian tumors of borderline malignancy at the Brigham and Women's Hospital and Massachusetts General Hospital between 1981 and 1990. Surgical staging was Stage I in 25 (66%) patients, Stage II in 2 (5%) patients, Stage III in 10 (26%) patients, and Stage IV in 1 (3%) patient. The mean sizes of mucinous and serous ovarian tumors were 21.9 and 10.3 cm, respectively (P = 0.0002). All 13 patients (100%) with mucinous tumors had Stage I disease, while 12 (50%) of 24 patients with serous tumors were Stage I. Combining all cell types, 10 (40%) of 25 patients with Stage I disease had an elevated preoperative CA-125 level, while 2 (100%) of 2, 9 (90%) of 10, and 1 (100%) of 1 patient with Stage II, III, and IV disease, respectively, had increased preoperative levels. Among patients with serous tumors, 3 (25%) of 12 Stage I patients had an elevated preoperative CA-125 level, while 11 (92%) of 12 Stage II-IV tumors had elevated levels (P less than 0.001). These data suggest that preoperative CA-125 level correlates with stage of disease in patients with serous borderline ovarian tumors.