We evaluated serurn CA-125 1evels in 122 patients with adnexal masses prospectively. The serum CA-125 levels were measured by immunoradiometric assay before laparotomy in 92 cases of benign ovarian turoor, 20 cases of malignant ovarian tumor, and 10 cases of pelvic abscess. in normal eontrol sixty-six apparently healthy women, the mean concentration of serum CA-125 level was 19. 2U/ml and false positive rate at cut-off limits 35U/ml and 65U/ml were l0.6% and 0%, The mean concentrations of preoperative surum CA-125 antigen in benign, malignant ovarian tumor and pelvic abscess were 32.8U/ml, 267. 1U/rnl, and 218. 5U/ml respectively, Among 92 patients with benign ovarian tumor, CA-125 levels above 35, and 65U/ml were detected in 23(25%), and 9(10,9%) cases of the patients respectively. Fifteen(75%) of 20 malignant ovarian tumors were associated with CA-125 levels above 35U/ml and 11 of 20 cases(55%) had Jevels that exceeded 65U/ml. Among 10 patients with pelvic abscess, CA-125 1evels greater than 35U/ml or 65U/ml were detected in 10 (100%) and nine (90%) patients. In 112 patients with benign and nine aet ovarian tumors, the positive rates of preoperative serum CA-126 levels were 76% sensitivity, 76% Speeifieity, 39.6% positive predietive value, 93,2% negative predictive value, and 75% diagnostie efficieney for a 35U/rnl cut-off and 55%, 89.1% 52.4%, 90.1%, and 83.0% respectively for a 65U/ml cut-off. These findings suggest that elevated serum CA-125 levels help differential diagnosis of ouarian rnalignancy from benign ovarian turnor preoperatively. The serum CA-125 levels seern to provide less information for early diagnosis of malignant ovarian tumors due to false negativity of early stage in ovarian malignaney but preoperative CA-125 leveJs ha.ve good correlation with clinical stage of ovarian mahgnancy. Accurate diagnosis of the malignant ovarian tumors may be difficult because pelvic ahscess showed especially high concentrations and fallse positive rate of preoperative serum CA-125, Therefore further evaluations of CA-125 levels in pelvic abscess is needed.