Ultrasonography (US) may demonstrate a diffuse reduction in thyroid echogenicity (low-amplitude echoes) in autoimmune thyroid disease (AITD), which includes chronic lymphocytic thyroiditis and Graves' disease, as well as in subacute thyroiditis. The reported occurrence of this finding in AITD varies from 19% to 95%. To assess the validity of diffuse reduction in thyroid echogenicity as a predictor of AITD, 3,077 patients referred for US of the thyroid were examined prospectively with regard to reduced versus normal thyroid echogenicity. The most frequent reasons for referral were goiter, thyroid dysfunction, neck discomfort, and/or difficulty in swallowing. Ultrasonography demonstrated diffuse reduction in thyroid echogenicity in 485 patients. Of these, 452 patients had available records of fine-needle aspiration biopsy (FNAB), and were included in the study. From the remaining patients, with normal thyroid echogenicity, 100 consecutive patients were selected as controls. In 411 of the 452 study patients (90.9%) there was at least one laboratory finding consistent with possible AITD: cytology indicating lymphocytic thyroiditis, 287 of 363 patients (79.1%) with diagnostic specimens; elevated levels of peroxidase antibodies (TPOAb), 225 of 337 (66.8%); elevated thyrotropin (TSH) levels, 290 of 450 (64.4%); or low TSH levels, 79 of 450 (17.6%). The final diagnosis was: chronic autoimmune (Hashimoto's) thyroiditis in 352 patients; Graves' disease in 47 patients; subacute (granulomatous) thyroiditis in 7 patients; toxic nodular goiter in 3 patients; and toxic adenoma in 2 patients. In the remaining 41 patients, those without laboratory results consistent with AITD, the final diagnosis was colloid goiter in 37 and thyroid cancer in 4 patients. In the 100 controls, laboratory results were consistent with possible AITD in 14 patients: elevated TPOAb levels in 5 of 49 patients with retrieved antibody results; lymphocytic thyroiditis in 2 patients; elevated TSH levels in 2 patients; and low TSH levels in 2 patients. In these controls, the final diagnosis was: chronic autoimmune thyroiditis in 7; toxic nodular goiter in 6 patients, and toxic adenoma in 1 patient. The corresponding positive and negative predictive values of reduced thyroid echogenicity as an indicator of AITD were 399 of 452 (88.3% [95% CI, 85% to 91%]), and 93 of 100 (93.0% [95% CI, 88% to 98%]), respectively. Thus, diffuse reduction in thyroid echogenicity was a valid predictor of AITD.