Cytological changes in the thyroid glands of 120 patients who had received I-131 treatment for hyperthyroidism were examined in detail. The cytological material was obtained by fine needle aspiration cytology. The patients were divided into four groups according to the amount of time elapsed after the I-131 treatment. The control groups were selected from euthyroid patients with hypoactive or non-functional solitary nodules and patients with Graves' disease who had not received I-131 therapy. The thyrocytes revealed degenerative changes in the immediate period after cessation of treatment. One year after the treatment some non-specific changes, such as oxyphilic metaplasia and regeneration, were seen. No malignant change has been observed clinically or cytologically during up to 20 years of follow up since receiving I-131.