Color flow Doppler sonography (CFDS) is gaining importance for the functional evaluation of the thyroid disorders. We aimed to determine the value of CFDS for the etiological diagnosis of hyperthyroidism. Fifty-five patients with hyperthyroidism (29 Graves' disease [GD] and 26 toxic adenoma [TA]), 24 patients with Hashimoto's thyroiditis (HT), and 39 euthyroid controls were included. Etiological diagnoses were carried out using standard methods. Conventional gray scale sonography was performed, followed by CFDS. Doppler patterns of the glands were scored and peak systolic velocity (PSV) measurements were obtained from intrathyroidal, perithyroidal, and perinodular vasculature. Vascular patterns were significantly more prominent, and the mean PSV values were significantly higher in the GD patients compared to the HT patients ( p < 0.001) and controls ( p < 0.001). Perinodular and intranodular signals and the mean perinodular PSV values were significantly higher in TAs compared to controls. CFDS could differentiate the untreated GD from the HT, which had similar gray scale findings. Hot nodules could also be differentiated from cold nodules with more prominent vascular patterns and significantly higher PSV values. As an inexpensive, fast, and noninvasive imaging procedure, CFDS could be helpful in the initial clinical evaluation and may avoid scintigraphy in a substantial number of thyrotoxic patients.