The clinical usefulness of thyroid ultrasonography in the evaluation of patients with autoimmune thyroiditis has been investigated. Thyroid ultrasonography was performed in 1184 consecutive patients attending our clinic, and the echo density of the thyroid parenchyma was evaluated with respect to that of normal thyroid tissue. Diffuse thyroid hypoechogenicity was found in 44 of 238 (18.5%) patients with autoimmune thyroiditis; the degree of hypoechogenicity was significantly correlated with the levels of circulating thyroid autoantibodies. Thyroid function was normal in all 194 patients with normal thyroid echogenicity, whereas hypothyroidism was found in 28 of 44 (63.6%) with reduced thyroid echogenicity. Included in this group were 8 patients, euthyroid at the first observation, who developed hypothyroidism over an 18-month follow-up period. None of the 133 patients with autoimmune thyroiditis and normal thyroid echogenicity followed for the same period of time became hypothyroid. Evidence of diffuse lymphocytic thyroiditis was obtained by histology after thyroidectomy (n = 10) or multiple fine needle aspiration cytology (n = 15) in 25 of the 44 patients with thyroid hypoechogenicity; on the other hand, focal thyroiditis was shown at histology in 8 patients who had normal thyroid echogenicity. In conclusion, diffuse low thyroid echogenicity was found in about 20% of patients with autoimmune thyroiditis. This echographic pattern is indicative of diffuse autoimmune involvement of the gland and is associated with or may predict the development of hypothyroidism.