The purpose of this study is to evaluate benign and/or malignant thyroid tumors with 201TI thyroid scan. We studied 76 cases of histologically verified thyroid tumors, all seen as cold nodules on the 123I thyroid scan. 201TI thyroid scan was performed 5-15 minutes (early scan) and 3-5 hours (delayed scan) after intravenous administration of 1.5-2.0 mCi of 201TI. In 35 (94.6%) of 36 malignant tumors (anaplastic carcinoma, six; papillary carcinoma 23; follicular carcinoma, five; epidermoid carcinoma, one; malignant lymphoma, 1) 201TI accumulated in the cold nodule of the 123I thyroid scan on both early and delayed scans. On the other hand, the delayed 201TI scan was negative in 35 out of 39 (89.7%) benign tumors. Employing early and delayed 201TI scans, we were able to differentiate most malignant thyroid tumors from those which were benign. False-negative and -positive cases are discussed.