This series retrospectively reviewed 3657 patients who received thyroid ultrasonography with fine-needle aspiration cytological (FNAC) examinations at Chang Gung Memorial Hospital during the period from January 1, 1993 to December 31, 1993. Thyroid ultrasonography studies were performed using a real-time ultrasonographic machine with a 10 MHz transducer. The aspirates were air dried and stained by the Romanowsky-based Liu method. Three hundred seventy-eight cases (10.3%) received surgical treatment after the ultrasonographic and FNAC examinations. Benign lesions were diagnosed in 269 patients. Thyroid malignancy was confirmed histopathologically in 109 cases including 76 papillary thyroid carcinomas, 17 follicular carcinomas, 5 medullary thyroid carcinomas, 3 anaplastic carcinomas, 3 Hu Β¨rthle cell carcinomas, and 3 lymphomas. Another 2 cases were metastatic cancer to thyroid. The results demonstrated that 28.8% of the surgically treated patients had histopathologically proven malignancies. The incidence of thyroid malignancy was 2.98% in this study. The sensitivity of the cytological diagnosis was 79.80% and the specificity was 98.66%. The positive predictive value was 96.34%. Negative predictive value was 91.70%. The false negative index was 20.20%. The diagnostic accuracy was 92.89%. Thyroid ultrasonography with the FNAC can provide high specificity and sensitivity in differentiating malignant lesions from benign.