We studied the utility of color Doppler ultrasonography in patients with subacute thyroiditis. Eighteen patients with subacute thyroiditis (SAT) with painful goiter and thyrotoxicosis underwent color Doppler ultrasonography during the acute and recovery stages of the disease. Thyroid vascularization in these patients was compared with that of 15 untreated patients with Graves' disease and 17 control subjects. During the acute stage of subacute thyroiditis, color Doppler ultrasonography showed low echogenicity without increased tissue vascularity in the affected swollen thyroid. In the recovery stage, color Doppler ultrasonography showed isoechogenicity with slightly increased vascularization. Vascularization became normal at 1 year follow-up time. In contrast, marked by increased vascularization was observed in patients with untreated Graves' disease. Color Doppler ultrasonography showed clear differences between SAT and Graves' disease patients. Vascularity was significantly correlated with serum free thyroxine (FT4) and thyrotropin (TSH) concentrations in the recovery stage (3 months after the initial ultrasonography). Color Doppler ultrasonography accurately visualized lesions without increased vascularity in the acute stage of SAT and lesions of slightly increased vascularity in the recovery stage. Color Doppler ultrasonography may be a useful, noninvasive, and rapid method for differentiating SAT from Graves' disease and for evaluating and monitoring the location and activity of lesions in SAT.