Twenty-one patients who underwent surgical treatment for thyrotoxicosis and who were found at operation to have thyroid cancer are presented. Sixteen had Graves' disease and 5 had toxic nodular goiter. The group with Graves' is compared with 110 euthyroid patients with thyroid cancer who underwent their initial surgery in the same time period and who were of the same age (+/- 1 yr) and sex as the patients with Graves' disease. None of the thyrotoxic patients died during follow-up of 2-24 yr or developed subsequent metastases. The 1 patient with a local lymph node metastasis has not shown evidence of recurrence. Hypoparathyroidism appeared as a complication in only 1 patient. The size of tumors in the patients with Graves' disease was significantly smaller than in the euthyroid group. The course of the disease in both the patients with Graves' disease and the thyrotoxic group as a whole was relatively benign. This series does not support the recent suggestions that thyroid cancer in patients with Graves' disease is more aggressive than in either patients with toxic nodular goiter or euthyroid subjects. Patients with Graves' disease and thyroid cancer should be treated identically to other patients with thyroid cancer. Therapy should consist of total thyroidectomy followed by a postoperative 131I scan. Residual tissue or metastases found on the scan should be ablated with 6 GBq 131I. The patient should receive a suppressive dose of T4.