PURPOSE: To evaluate the usefulness of volume change after thyroxine suppression therapy in predicting the malignancy of thyroid nodules. MATERIALS AND METHODS: We analyzed 28 cases of thyroid nodules diagnosed as benign by aspiration cytology or biopsy before thyroxine suppression therapy and which did not decrease more than 50% in volume after therapy. Using Ultrasonography we measured the volume of nodules before and after thyroxine suppression therapy and then determined volume change by calculating nodule volume suppression rate (NVSR) and comparing this beween benign and malignant nodules. All cases were surgically confirmed. N V S R ( % ) = nodule volume after treatment x 1 0 0 nodule volume before treatment RESULTS: Seventeen (60.7%) of 28 thyroid nodules which showed less than 50 % NVSR after thyroxine therapy were shown by surgical resection to be malignant, while 11(39.3 %) were benign. The malignant cases were papillary carcinoma (n=9) and follicular carcinoma (n=8), while benign cases were either follicular adenoma (n=7) or adenomatous hyperplasia (n=4). The mean volume of malignant thyroid nodules was 5.2 +/- 4 .5cm3 before thyroxine suppression therapy and 5.9 +/- 4 .5 cm3 after therapy, and that of benign nodules, 12.1 +/- 10.3 cm3 before therapy and 10.1 +/- 9.9 cm3 after. NVSR was 127.2 +/- 46.2 % in malignant nodul es and 79.4 +/- 21.0% in benign nodules, with no significant difference between the two groups (P < 0.05). CONCLUSION: Although further studies may be required, fine needle aspiration cytology or biopsy showed that among benign thyroid nodules which did not decrease more than 50 % in volume after thyroid therapy, the incidence of malignancy revealed by surgical resection was more than 60 %. Surgical exporation of these nodules is therefore highly recommended.