We studied preoperative CA-125 levels of 123 patients with borderline ovarian tumors (BOTs) and performed an analysis with data of earlier published studies. CA-125 levels were compared according to histology and stage of disease. Preoperative serum CA-125 levels were significantly higher for patients with advanced stage (median, 181 U/mL; range, 413 U/mL) compared with patients with stage I (median, 28 U/mL; range, 1123 U/mL) BOTs and for patients with serous (median, 59 U/mL; range, 1119 U/mL) compared with patients with mucinous (median, 25 U/mL; range, 371 U/mL) BOTs (both P < 0.001, Mann-Whitney U test). A pooled analysis of 3 studies and the present study showed positive rates of CA-125 (value >35 U/mL) in 171 (53%) of 325 patients with BOTs. Positive rates were more often found in patients with serous (67%) compared with patients with mucinous BOTs (39%) and in patients with advanced stage (83%) compared with patients with stage I BOTs (47%) (both P < 0.001, Pearson chi(2) test). This main effect was also found for each individual study of the pooled analysis. From a clinical perspective, we believe, on base of the results of this study and the literature, that preoperative discrimination using CA-125 level is especially difficult between patients with stage I ovarian cancer and the group of patients with serous and/or advanced-stage BOTs.