Thyroid hypoechogenicity at ultrasound is a characteristic of autoimmune thyroid diseases, with an overlap of this echographic pattern in patients affected by Graves' disease or Hashimoto's thyroiditis. Aim of the present paper was to study the thyroid blood flow (TBF) by color-flow doppler (CFD) and peak systolic velocity (PSV) at the inferior thyroid artery in 37 Graves' and 45 goitrous Hashimoto's thyroiditis patients. CFD pattern was defined as normal (or type 0): TBF limited to peripheral thyroid arteries (PSV = 17.7 +/- 3 cm/sec, mean +/- SD); type I: TBF mildly increased; type II: TBF clearly increased; type III: TBF markedly increased. The CFD was in direct relationship to the PSV. Out of 18 patients with Graves' disease and untreated active hyperthyroidism CFD pattern was type III in 17 and type II in 1. The PSV was 42.1 +/- 15 cm/sec. In 17 patients euthyroid under methimazole, the CFD pattern was type 0 in 3 (17%) type I in 5 (30%), type II in 5 (30%), type III in 4 (23%). In this group of Graves' patients the PSV was 36 +/- 14 cm/sec. In two patients, hypothyroid after radioiodine treatment, the CFD pattern was type 0 in 1 and type I in 1. In the group of Hashimoto's patients TBF was in no relationship with thyroid status or treatment and was type 0 in 22 (49%), type I in 20 (44%), type II in 3 (7%), while none had type III CFD pattern. Thyroid hypoechogenicity at ultrasound was present in 32/37 (86%) Graves' and 41/45 (91%) Hashimoto's patients. All the four patients with Hashimoto's thyroiditis and normal thyroid ultrasound pattern had also a normal CFD pattern, while 4/5 patients with Graves' disease and normal echographic pattern had an increased TBF. In conclusion, a diffusely increased thyroid blood flow is pathognomonic of untreated Graves' disease and an abnormal CFD pattern identifies the majority of Graves' patients with a normal thyroid ultrasound pattern. Thus, CFD sonography may be useful in distinguishing patients with Graves' disease and Hashimoto's thyroiditis having a similar thyroid echographic pattern at ultrasound.