The pathogenesis of thyroid carcinoma in Graves' goiter is still obscure and the methods for preoperative diagnosis of such carcinomas is not well established. We studied the incidence, clinical features, and pathological findings of thyroid carcinoma in Graves' goiter. From October, 1983 to September, 1985, a total of 739 patients with Graves' disease underwent subtotal thyroidectomy at Ito Hospital, Tokyo. All of these patients underwent roentgenography of the neck before surgery. Thyroid carcinoma was revealed in the resected specimen in 15 (2.0%) of 739 patients. During the same period, another 4 patients underwent surgery for thyroid carcinoma who had had Graves' disease previously and these 4 cases were included in the present study. The incidence of thyroid carcinoma associated with Graves' disease was 2.6% (19 of 743 cases). Histological examination revealed 15 papillary and 4 follicular carcinomas. The size of the carcinoma foci was 13.8 +/- 15.6 mm in diameter (range: 1-60 mm), 10 (52.6%) foci being 10 mm or greater. Invasive growth into the surrounding thyroid tissue was predominant and regional lymph node metastasis was noted in all 6 patients who underwent cervical dissection. Preoperative roentgenography revealed calcification in 5 (26.3%) of 19 cases. Our present study indicates that thyroid carcinoma in Graves' goiter may show markedly invasive growth with lymph node metastasis even though the primary tumor is small in size, and it is suggested that the detection of calcification may serve as a part of the diagnostic measures when the carcinoma focus is difficult to palpate in the diffusely-enlarged Graves' goiter.