Progression from C-cell hyperplasia (CCH) to medullary thyroid carcinoma (MTC) has been demonstrated to date only in familial forms, whereas in nonfamilial MTC, such hypothesis is suggested by the rare concurrence of both lesions, although no epidemiological and molecular data are available to prove or disprove this event. Therefore, the clinical management of patients with sporadic CCH is controversial. To evaluate the malignant potential of sporadic CCHs, pure laser-microdissected C-cell populations of 24 CCH cases, either reactive or associated with nonfamilial MTC, were analyzed for MTC-associated protein neural cell adhesion molecule expression and RET point mutations in exons 10, 11, 15, and 16, by using immunohistochemistry and polymerase chain reaction-single-strand conformation polymorphism/heteroduplex electrophoresis/direct sequencing, respectively. No RET mutations were found in any of the 24 CCH cases, whereas M918T mutation was detected in three concomitant MTCs. Neural cell adhesion molecule was immunoreactive in the majority of CCH associated with MTC even in the absence of morphological atypia, but not in reactive forms. The absence of RET alterations in all cases of CCH examined supports the hypothesis that the development of MTC is independent of pre-existing CCH in the nonfamilial setting; thus, sporadic CCH should not be considered a risk factor for nonfamilial MTC.