PURPOSE: To assess the usefulness of transvaginal sonography(TVS) in the detection of normal contralateral ovary and disease involvement of contralateral ovary in the patients with ovarian tumor. MATERIALS AND METHODS: We compared sonographic findings with histopathologic findings of the contralateral ovary retrospectively in 87 patients, who underwent preoperative ultrasonography and laparotomy for ovarian tumor for recent 4 years. RESULTS: Abnormality of the contralateral ovary was confirmed in 49 (56.3%) of 87 patients. The pathologic diagnoses of contralateral ovarian lesions were bilateral involvement of the same disease in 39 patients, different tumor in four patients and non-tumorous lesion in six patients. Abnormal TVS findings of the contralateral ovary were detected in 34 of 49 patients, which shows diagnostic accuracy of 82.8%. The sensitivity and specificity were 69.4% and 100%, respectively. 15 cases which were not diagnosed by ultrasound were bilateral involvement of the same disease in 10 cases (1 serous cystadenoma, 2 cystadenocarcinoma with low malignant potential, 1 brenner tumor, 1 metastatic endometrioid cancer, 1metastasis, 4 teratoma) and different lesions in the remaining 5 patients (2 endosalpingiosis, 1 surface inclusion cyst, 2 tuboovarian cyst). CONCLUSION: Ultrasound of the contralateral ovary in the patients with ovarian tumor shows low to a moderate degree sensitivity and accuracy. So, more intensive and targeted evaluation of contralateral ovary is needed for the more accurate diagnosis and proper treatment.