Twenty-two of 251 patients with differentiated thyroid carcinoma suffered from or had a history of hyperthyroidism. They were hyperthyroid with a diffuse goitre (N = 4), a diffuse goitre with a cold nodule (N = 10), a multinodular goitre (N = 6), and an autonomous adenoma (N = 2). Among the 22 patients, more than one fourth had an occult thyroid carcinoma with a diameter of 1 cm or less, those with the papillary tumour types, less frequently had lymph node metastases than the total group of patients with papillary carcinomas (13.3 vs 35.6%). The clinical courses of the 22 patients resembled those of the other thyroid carcinoma patients whose age and initial findings were comparable. In 643 patients who underwent surgery for hyperthyroidism the incidence of thyroid carcinoma was 2.3%. The increase in coincidence of hyperthyroidism and thyroid carcinoma repeatedly reported in recent years is probably ascribable primarily to extensive and improved diagnostics and not to a direct connection between hyperthyroidism and development of thyroid carcinoma. On the other hand, our findings do confirm that, even in the presence of hyperthyroidism, all thyroid nodules require careful diagnostics for exclusion of malignancy.