Findings in seven cytologic specimens obtained with fine-needle aspiration biopsy were described in three patients in whom thyroid nodules developed after iodine 131 treatment of hyperthyroidism. In two patients, the presence of large bizarre cells with hyperchromatic nuclei, coarse chromatin, and prominent nucleoli suggested the possibility of malignancy, and in particular, anaplastic carcinoma. These bizarre nuclear changes were diffuse in one patient and focal in the other. In the third patient, nuclei with chromatin clearing, grooves, and nuclear pseudoinclusions suggested papillary carcinoma. At follow-up, lesions were benign in all three patients. We conclude that nuclear changes suggestive of either anaplastic carcinoma or papillary carcinoma may be seen in thyroid nodules after 131I treatment. Clues to the benign nature of these nodules include architectural arrangement of cells in cohesive clusters and flat sheets; presence of lymphocytes, histiocytes, colloid, cytoplasmic vacuoles, and Hürthle cells; and a clinical history of treatment with radioactive iodine.