Fifty two patients with cold thyroid nodule demonstrated on the thyroid scan were imaged with 201Tl which were given intravenously as thallium chloride in dose of 2 mCi. Thirty nine of 52 patients were confirmed and investigated whether 201Tl concentrated or not. Fourteen of 15 (93 percent) thyroid carcinomas, 5 of 17 (29 percent) thyroid adenomas, 1 of 2 adenomatous goiters and all of 5 of chronic thyroiditis were visualized as positive with 201 Tl. One thyroid carcinoma did not concentrate 201Tl which was confirmed to have cystic degeneration. Of the 19 benign cold thyroid nodules except chronic thyroiditis 6 were positively visualized with 201Tl. However, 201Tl did not accumulate in the other 13 benign nodules, 11 out of which were confirmed to have cystic degeneration. The data suggests that if the thyroid nodule is found to have negative accumulation of 201Tl, malignancy can be ruled out except a small microscopical lesion.