We used real-time ultrasonography to examine 60 patients with autoimmune thyroiditis, then correlated the ultrasonic pictures with thyroid function, thyroid autoantibodies and fine needle aspiration cytology. In these 60 patients, 45 (75%) showed diffuse goiter, 6 (10%) showed multinodular goiter, and 9 (15%) had a solitary thyroid nodule sonographically. One of the 9 patients with a solitary nodule was a case of autoimmune thyroiditis combined with papillary carcinoma. The echogenicity of the thyroid was more than, the same as, or less than that of the adjacent muscles in 17, 22, and 21 patients, respectively. The groups were classified as hyperechoic, isoechoic, and hypoechoic, respectively. The mean serum T4 level was significantly lower in the hypoechoic group than in the hyperechoic or isoechoic groups (p less than 0.01 and p less than 0.05, respectively), and the incidence of hypothyroidism was significantly higher in the hypoechoic group than in the hyperechoic or isoechoic groups (p less than 0.001 and p less than 0.005, respectively). In addition, high titers of the antithyroid microsomal antibody (greater than or equal to 1280) were present more frequently in the hypoechoic group than in the hyperechoic or isoechoic groups (p less than 0.01 and p less than 0.05, respectively). There was no significant correlation between the cytomorphology and echogenicity of the thyroid in these cases. We conclude that sonography has two major uses in evaluating autoimmune thyroiditis: First, it is useful in excluding the coexistence of thyroid nodules; and second, marked hypoechogenicity of the thyroid implies an active cytotoxic autoimmune process and possibly a hypothyroid state.