Ultrasonography is a first line method in thyroid imaging. The aim of this study was to evaluate the usefulness of thyroid ultrasonography in the diagnosis of thyroid autoimmune diseases (TAD): Graves' disease (GD) and autoimmune thyroiditis (AT)--Hashimoto's and atrophic thyroiditis. Ultrasonography was performed with a 7.5 MHz transducer in 1033 consecutive patients. The excluding criteria was nodular goiter, irrespective of its function. Thyroid volume was calculated using the formula of a rotating ellipsoid. Hypoechogenicity was appreciated using a qualitative scale: 1 = absent, 2 = mild, 3 = moderate, 4 = marked. In patients with TAD thyroid hypoechogenicity was significantly higher than in normal subjects (2.5 +/- 1 in GD and 2.7 +/- 1 in AT, vs. 1.2 +/- 0.6 in normal subjects, p < 0.0001). The specificity of the test in identifying TAD was 91.4%, the sensitivity was 68.6% for GD and 80.8% for AT, positive predictive value was 91.5%, and negative predictive value 91.5%. Ultrasonography is useful in identifying patients with probable autoimmune thyroid diseases, being useful in epidemiological studies and having an important contribution in confirmation of the diagnosis.