Ultrasound screening in 9 and 21 members, respectively, of two families affected by familial C-cell carcinoma, as well as sonographic findings in 6 patients with sporadic medullary carcinoma of the thyroid (MCT), are reported. Unilateral and bilateral tumor nodules were identified by sonography in 12 of a total of 13 patients with MCT (n = 9) or local tumor recurrence following thyroidectomy (n = 4); one carcinoma, 3 X 6 mm in diameter, was missed. Nine of the 12 (75%) were clinically occult, nonpalpable C-cell carcinomas. The smallest occult MCT correctly diagnosed by ultrasound had a diameter of 4 mm. A positive correlation was found between the sonographically determined tumor mass (number/size of nodules/metastases) and the plasma calcitonin level. Pheochromocytomas were diagnosed by ultrasound as part of a multiple endocrine neoplasia (MEN IIa) in two patients with MCT. The echomorphologic findings of the intrathyroid C-cell carcinomas and their metastases display characteristic but nonspecific features. Sonographic findings on these tumors should therefore not be interpreted without consideration of plasma calcitonin assays.