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The objectives of this study were: 1) to evaluate the role of color-Doppler ultrasonography (CDU) assessment of thyroid vascularity, measuring the peak systolic velocity (PSV) at the level of the inferior thyroid artery, and the intrathyroid vascularization in Graves' diseas; 2) to evaluate the role of contrast agent administration in predicting the relapse of hyperthyroidism or the biological activity of the disease after withdrawal of antithyroid drugs.The study included 74 Graves' patients (59 F/ 15 M; mean age 45 years; range 23-71). Graves' disease was diagnosed according to the usual clinical and laboratory criteria. On the basis of the clinical and biochemical findings we divided Graves' patients into 4 different groups. Treatment was continued for at least 12 months, CDU examination was carried out after discontinuing therapy. Eight patients showed a relapse of hyperthyroidism within 5 months after suspension of therapy. In all cases the evaluation of intraparenchymal vascularization and PSV at the level of the inferior thyroid artery in basal conditions was followed by administration of contrast agent (Levovist, 300 mg/ml), with slow infusion (2 ml/min) to avoid blooming artifact. Intraparenchymal vascularization was classified into 4 patterns according to Vitti et al.The value of the peak systolic velocity (PSV) at the level of the inferior thyroid artery was the best predictor of relapse. A value higher than 40 cm/sec was present in all the patients that showed relapse and only in two patients with stable remission. Administration of contrast agent is important to evaluate the biological activity of the disease. In the 5 patients exhibiting slightly increased vascularization after contrast agent administration we could assume the clinico-pathological recovery.CDU study of thyroid vascularization, based on the measurement of PSV at the level of the inferior thyroid artery and on the response to contrast agent administration is useful to distinguish three groups of patients: A) PSV40 cm/sec with pattern III for at least 10 minutes from the beginning of the contrast agent administration (High risk of relapse); B) PSV40 cm/sec with pattern II or III after contrast agent administration (Biological activity of the disease-Thyroiditis); C) PSV40 cm/sec with Pattern I after contrast agent administration (Poor biological activity of the disease-recovery).