Moderately differentiated neuroendocrine carcinoma/atypical carcinoid tumor is the most common nonsquamous malignancy in the larynx; however, due to morphologic overlap and calcitonin immunoreactivity, it can be difficult to distinguish from thyroid medullary carcinoma. Currently, low serum calcitonin is the most reliable means for distinguishing primary laryngeal moderately differentiated neuroendocrine carcinoma from metastatic medullary carcinoma. Thyroid transcription factor-1 (TTF-1) is positive in at least 80% of medullary carcinomas, but has not been evaluated in laryngeal moderately differentiated neuroendocrine carcinomas. Additionally, it has been suggested that p53 is positive in laryngeal moderately differentiated neuroendocrine carcinomas and negative in other neuroendocrine tumors, but this has not been validated. The purpose of this study was to determine if the immunohistochemical markers TTF-1 and p53 could be used to discriminate between laryngeal moderately differentiated neuroendocrine carcinomas and thyroid medullary carcinomas. Eight laryngeal moderately differentiated neuroendocrine carcinomas and 10 thyroid medullary carcinomas were identified from the archival files of the BWH and MGH Pathology Departments. Hematoxylin and eosin slides were reviewed, and immunohistochemistry was performed using antibodies to calcitonin, TTF-1, and p53. Calcitonin immunohistochemistry demonstrated immunoreactivity in 100% of laryngeal moderately differentiated neuroendocrine carcinomas (N=8) and 100% of thyroid medullary carcinomas (N=10). There was weak, focal immunoreactivity with TTF-1 in one of eight (13%) laryngeal moderately differentiated neuroendocrine carcinomas, whereas nine of ten (90%) medullary carcinomas were positive for TTF-1, with strong diffuse staining in seven of these cases (78%). p53 was positive in three of six (50%) laryngeal moderately differentiated neuroendocrine carcinomas, and three of ten (30%) medullary carcinomas. Our data demonstrate that immunoreactivity for TTF-1, but not calcitonin or p53, may be helpful in distinguishing laryngeal moderately differentiated neuroendocrine carcinoma and thyroid medullary carcinoma. In particular, diffuse and/or strong TTF-1 immunoreactivity favors a diagnosis of primary thyroid medullary carcinoma over laryngeal moderately differentiated neuroendocrine carcinoma.