Abstract Background. Medullary thyroid carcinoma (MTC), a tumor arising from calcitonin-secreting C cells, appears in either a sporadic nonfamilial or a hereditary form as a component of a multiple endocrine neoplasia syndrome or familial non-multiple endocrine neoplasia MTC. Screening kindred of a patient with familial MTC to detect MTC in the early curable state is standard practice. Opinions conflict about whether it is necessary to screen relatives of patients with apparently nonfamilial MTC to exclude hereditary disease or whether the clinicopathologicfeatures can differentiate between the two forms. Methods. Clinically well kindred of a patient with MTC that was histopathologically characteristic of the sporadic type were screened for hereditary disease by measurement of plasma levels of basal and stimulated calcitonin. Results. Three of four immediate relatives tested positive for excessive calcitonin secretion and underwent thyroidectomy. All had C-cell hyperplasia, the premalignant phase of MTC. Conclusions. The patient with apparently sporadic (nonfamilial) MTC was clearly an index case of familial disease. We conclude that clinical presentation and histopathologic examination are not adequate to reliably exclude hereditary MTC. Until genetic markers are readily available to distinguish between sporadic and familial forms, biochemical screening should be done in primary relatives of all patients with newly detected MTC.