A quantitative systematic review was performed to estimate the accuracy of CA 125 assay in the diagnosis of ovarian tumors. Studies that evaluated CA 125 levels for the diagnosis of ovarian tumors and compared them with paraffin-embedded sections as the diagnostic standard were included. Seventeen studies were analyzed, which included 2374 women. The pooled sensitivity for the diagnosis of borderline tumors or ovarian cancer was 0.80 (I.C. 95% 0.76-0.82) and the specificity was 0.75 (I.C. 95% 0.73-0.77). The diagnostic odds ratio for ovarian cancer and borderline lesions vs. benign lesions was 21.2 (95% C.I., 12-37). Summary receiver operating characteristic curves were constructed due to heterogeneity in the diagnostic odds ratio. For malignant and borderline ovarian tumors vs. benign lesions the area under the curve was 0.8877. A CA 125 level of >or= 35 U/ml is a useful preoperative test for predicting the benign or malignant nature of pelvic masses. The accuracy of CA 125 in the diagnosis of ovarian tumors is high and very important in helping the surgeon to decide what kind of surgery should be performed.