Review of medical records in 600 consecutive cases of primary hyperparathyroidism revealed 10 patients with a documented history of iodine 131 (131I) treatment. In seven cases 131I had been given because of Graves' disease and in three cases for ablation of thyroid remnants after tumor operations. All but one of the patients were women. Their age at the time of 131I treatment ranged from 21 to 72 years, and the interval to detection of hypercalcemia was between 3 and 27 years. It is noteworthy that all patients treated for Graves' disease had absorbed radiation doses large enough to cause permanent hypothyroidism, and half of them showed complete absence of the thyroid gland at subsequent operation for hyperparathyroidism. Furthermore, parathyroid adenomas had developed at the sites of thyroid remnants in cases with 131I ablation after tumor operations. Our findings support other observations indicating that not only external radiation but also radiation from 131I is a risk factor for development of hyperparathyroidism, and it is emphasized that age at the time of radiation treatment may be of decisive importance in this context.