Purpose: To evaluate the usefulness of ultrasonogram as a preoperative diagnostic tool in thyroid nodular diseases, this study was carried out. Materials and Methods: From January 1998 to December 1999, 51 patients who underwent thyroidectomy were analyzed retrospectively. We compared the finally histopathological results to ultrasonographical findings such as internal consistency, multiplicity of nodules, nodular ehogenicity, nodular capsule or margin, calcification of nodules. Results: There were 47 females and 4 males with 25 benign tumor, 23 malignant tumor and 3 occult carcinoma in histopathological diagnosis. The solid tumors in ultrasonography carried a probability of malignancy as 66.7%(16/24 cases) whereas cystic or mixed tumors as 16.7%(1/6cases) or 23.8%(5/21cases) (p=0.006). The single nodular diseases carried a high probability of malignancy as 50%(13/26cases) whereas multiple diseases as 28.6%(6/21cases). The hypoechogenicity of thyroid nodular disease showed a probability of malignancy as 60%(9/15cases) whereas mixed-echogenicity as 36.4%(4/11cases). The nodules with poorly-defined margin in ultrasonographic findings showed higher probability of malignancy as 63.6% (7/11cases) than the nodules with well -defined margin as 26.5%(9/34 cases) (p=0.025). The nodules with calcification in ultrasonographic findings were represented to high probability of malignancy as 70.6%(12/17cases) compared to those without calcification as 29.4%(10/34cases) (p=0.005). The differency between ultrasonic and histopathological diagnosis was high in solid nodules(33%), 3-4cm sized nodules (28.6%) and mixed echogenecity(27.3%) whereas low in complex nodules with cystic and solid nature(4.8%), 2-3 cm sized nodules(8.3%) and pooly defined margin(9.1%). The accuracies of sonography in differentiating malignacy from benign thyroid nodules were 7.1% of false positivity, 39.1% of false negativity, 60.9% of sensitivity, 92.9% of specificity and 78.4% of accuracy. Conclusion: Sonographic examination was relatively excellent test as a preoperative diagnostic tool in thyroid nodular diseases when detailed checklists were applied such as internal consistency, multiplicity of nodules, nodular ehogenicity, nodular capsule or margin and calcification of nodules.