An abnormal thyroid echographic pattern characterized by a diffuse low echogenicity has been described in Hashimoto's thyroiditis and Graves' disease. The aim of the present work was to study the relationship between thyroid hypoechogenicity and the outcome of treatment for hyperthyroidism with antithyroid drugs in patients with Graves' disease. The study group included 105 patients who underwent a course of methimazole treatment. Thyroid ultrasonography was carried out at diagnosis, and autoantibodies to thyrotropin receptor (TR-ab) were measured at the end of treatment. During the follow-up after methimazole treatment, 87/105 (83%) patients had relapse of hyperthyroidism and 18/105 (17%) were in remission. Recurrence of hyperthyroidism occurred in 71/76 (93%) patients with thyroid hypoechogenicity and in 16/29 (55%) of those with normal thyroid echogenicity (chi 2 = 19.0; p less than 0.0001). Positive TR-ab values at the end of methimazole treatment were found in 59/76 (78%) patients with thyroid hypoechogenicity and in 12/29 (41%) patients with normal thyroid echogenicity (chi 2 = 10.9; p less than 0.0001). Sixty-five/87 (74%) patients with relapse of hyperthyroidism and 6/18 (33%) of those who remained euthyroid were TR-ab-positive at the end of methimazole treatment (chi 2 = 9.8; p less than 0.002). The finding of thyroid hypoechogenicity at diagnosis had higher specificity (0.81) and sensitivity (0.72) with respect to TR-ab positivity at the end of methimazole treatment (0.74 and 0.66 respectively) for the prediction of relapse of hyperthyroidism. Therefore, the evaluation of thyroid echographic pattern can be considered a useful prognostic tool in patients with Graves' disease.