It is well established that external radiation to the neck area can act as an initiating factor in the development of hyperparathyroidism (HPT). In the present work the incidence of HPT was studied in a series of 125 patients treated with 131I for thyrotoxicosis and in untreated controls matched for age and sex. No increased incidence of HPT was found among the radioiodine treated patients. The time from treatment to follow-up was 21 years on the average. The most probable reason for the lack of an effect on the HPT-incidence is that variable and mostly low doses of radiation with low dose rates are absorbed in the parathyroids after 131I-treatment for thyrotoxicosis.