Iodine-131 is used to detect and treat functioning lesions of papillary and follicular carcinoma of the thyroid. False-positive images although rare, do occur, with major categories being 1) body secretions, 2) pathologic transudates and inflammation, 3) nonspecific mediastinal uptake, and 4) neoplasms of nonthyroidal origin. The authors present eight cases that fall into the first three categories, including three types not previously reported--a lymphoepithelial cyst, a scrotal hydrocele, and a skin burn. The classification of mediastinal uptake as false positive is tentative. High-dose images and those exploiting therapeutic doses have an increased detection rate for both true and spurious lesions. Appropriate caution is advised in the interpretation of I-131 images.