PURPOSE: To evaluate useful findings and diagnostic value of TI-201 thyroid scan in differentiating benign from malignant nodules. MATERIAL AND METHOD: We studied 77 cold thyroid nodules proven histologically(27 malignat and 50 benign). Early (5--15rain) and delayed images(3--5hours) were obtained after intravenous injection of thallium 201. In these nodules, we retrospectively analyzed the degree of TI-201 uptake in early and delayed images, histopathologic type, size, and presence or absence of cystic change in the sonograms of 22 malignant nodules. RESULTS: Useful finding for diagnosis of malignant nodules was strong uptake of TI-201 in early and delayed images(specificity:98%, sensitivity:63%, positive predictive value:94.4%). Useful finding for benign nodules was no uptake of TI-201 in delayed image(specificity :88.9%, sensitivity :68%, positive predictive value :91.9%). The accuracy of TI-201 thyroid scan in differentiating benign from malignant nodules was 66.2%. The nodules with strong TI-201 uptake in early image and low TI-201 uptake in delayed image were malignant in 29.4%. Cystic changes were found in 40% of malignant nodules with atypical TI-201 uptake. TI-201 thyroid scan showed high specificity in follicutar neoplasm and adenomatous goiter in which differentiation of benignancy and malignancy is difficult with only cytologic examination. CONCLUSION: We consider that TI-201 thyroid scan is valuable in differentiating benign from malignant nodules and when combined with fine needle aspiration and ultrasound examination, it will enable more accurate differential diagnosis between benign and malignant thyroid nodules.