The effect of 131I therapy was examined in 13 thyroid glands affected by Graves' disease 3 to 29 years after irradiation for hyperthyroidism. All of the patients had clinically palpable thyroid nodules. Two patients were in a latent hypothyroid, 6 in a euthyroid and 5 in a hyperthyroid state. The microscopic changes in the thyroids showed a pattern of multiple adenomatous nodules with cystic changes, marked oxyphilic cell changes with nuclear atypism and various degrees of chronic thyroiditis. Immunohistochemical staining for TG and T4 was negative to mildly positive in these oxyphilic cells and entirely negative for EGF and CEA. The DNA ploidy pattern was diploid pattern in 6 cases. One papillary-type microcarcinoma occurred, but there was no evidence of a relationship between the tumor and the irradiation. The pathologic findings in Graves' thyroid gland after 131I therapy are not specific, but pathologists should differentiate this lesion from adenomatous goiter, which occurs with no apparent cause, or from thyroid carcinoma because of the marked nuclear atypism of this lesion.