Bilateral medullary carcinoma of the thyroid (MCT) was discovered in a symptomless patient of a high-risk MCT family. Raised serum calcitonin responding abnormally to pentagastrin led to a thyroidectomy. Grossly, the thyroid showed one nodule in the upper third of each lobe. Light microscopy revealed typical MCT with amyloid deposits. Nontumoral thyroid tissue showed C cell parafollicular hyperplasia. Electron microscopy detected intracellular secretory granules (mean diameter, 150 nm) and typical amyloid fibrills. Immunochemistry revealed numerous calcitonin immunoreactive cells in the nodules, in normal para and intrafollicular C-cells and hyperplastic C-cells. Somatostatin and ACTH were detected in certain tumor cells, but not in normal and hyperplastic C-cells. MCT hormonal production potential is discussed.