Sonographic findings in 109 thyroid nodules in 104 patients with Hashimoto thyroiditis were retrospectively analyzed. Sixty patients underwent ultrasound (US)--guided fine-needle aspiration (FNA) biopsy (65 nodules, 24 palpable, 27 surgically confirmed), 14 of whom also underwent palpation-guided FNA (14 nodules, all palpable, all surgically confirmed). Forty-four patients (44 nodules, 25 palpable) underwent diagnostic US followed up with surgery. US-guided FNA helped differentiate between benign (n = 11) and malignant (n = 16) diseases (sensitivity, 100%). Two papillary carcinomas were falsely diagnosed as Hashimoto thyroiditis with palpation-guided FNA. Eight malignancies were not palpable, and correct diagnosis was obtained with US-guided FNA. Hyperechoic nodules were usually benign, and isoechoic nodules had a low frequency (13%) of malignancy. Most carcinomas were found in hypoechoic masses, and almost all lymphomas were found in markedly hypoechoic masses. Lymphoma was indistinguishable from pseudotumor in Hashimoto thyroiditis or adenomatous hyperplasia at US. US-guided FNA is helpful in making the histologic diagnosis in thyroid nodules associated with Hashimoto thyroiditis. US-guided FNA is clinically beneficial because it can help select patients who need surgery, avoiding unnecessary surgery for patients with nonneoplastic disorders.