During an 8-year period ending 1981, 372 patients were treated surgically for hyperparathyroidism (HPT) only, 93 for nonmedullary thyroid carcinoma only, and 19 for combined disease (patients traced by follow-up excluded). Seventy-three of these 484 patients (15.1%) had a history of irradiation to the neck region. When calculated for both the HPT and thyroid carcinoma patients there was a significantly increased incidence of associated HPT and thyroid carcinoma among irradiated individuals (P less than 0.001). In irradiated patients with combined disease the increase in serum calcium level was of the same magnitude as in the patient with HPT only, and the distribution according to stage of carcinomas was similar to that in the nonirradiated patients with thyroid carcinoma only. The present findings are in accordance with the hypothesis that radiation can serve as an etiologic factor in the development of associated thyroid carcinoma and HPT.