C-cell hyperplasia precedes the development of medullary thyroid carcinoma in multiple endocrine neoplasia type 2A (MEN2A). Identification of abnormal calcitonin levels after a provocative stimulus is a technique that has been widely used to diagnose this preneoplastic condition in an early stage during the development of medullary thyroid carcinoma, when total thyroidectomy is likely to be curative. In a MEN2A kindred, we identified seven individuals with abnormal calcitonin test results, whose carrier state was questionable. Five of these people were thyroidectomized, and C-cell hyperplasia was diagnosed. Four of these individuals were the offspring of a mother who is at risk for the development of MEN2A but who has had normal calcitonin test results throughout the years and of a father who is not at risk but who has had abnormal test results over a period of 10 years, without evidence of progressive elevation. None of these people developed other manifestations of MEN2A. DNA analysis using markers linked to the MEN2A gene demonstrated, with > 99% likelihood, that none of the individuals who could be genotyped was a gene carrier. C-cell hyperplasia due to some mechanism other than the presence of the MEN2A gene may also occur in MEN2A kindreds. DNA analysis offers an important additional tool for proper diagnosis in the clinical management of MEN2A families.