The clinical behavior and outcome was evaluated in 21 nonoccult differentiated thyroid carcinomas occurring in Graves' patients during the period 1982-94 and compared with that of matched tumors occurring in euthyroid controls (n = 70). At surgery, patients with Graves' disease showed distant metastases more frequently than euthyroid patients (3/21 = 14.3% vs. 1/70 = 1.4%, P = 0.0556). Graves' patients also showed a significantly higher cumulative risk of recurrent/progressive distant metastases or total adverse events (odd ratios = 3.14 and 2.07, respectively) as compared with euthyroid patients. At the last follow-up visit, persistence of distant metastases was also more frequent in the Graves' group (P = 0.007), although the cumulative individual dose of radioiodine administered was higher than in the control group (median dose = 805 mCi vs. 350 mCi). Two patients died in the Graves' group vs. none in the control group. Circulating thyroid stimulating antibodies were present in all patients but one and persisted as long as signs of disease were evident. These findings indicate that differentiated thyroid carcinomas in patients with Graves' disease are more aggressive than those occurring in matched euthyroid controls and should, therefore, be managed accordingly.