Recently, Orthopedia Homeobox (OTP) was described as a prognostic marker for pulmonary carcinoid tumors; however, little is known about the function and distribution pattern of this transcription factor in normal organs/tissues and in tumors. Consequently, OTP expression was investigated in a variety of tumors, with special interest in pulmonary and nonpulmonary neuroendocrine tumors (NETs) and high-grade neuroendocrine carcinomas. OTP immunohistochemical analysis was performed on a total of 162 pulmonary carcinoid tumors, 31 pulmonary neuroendocrine hyperplasias, 104 pulmonary high-grade neuroendocrine carcinomas (large cell neuroendocrine and small cell neuroendocrine), 102 nonpulmonary NETs (G1/G2 NETs, small cell and large cell neuroendocrine carcinomas, and Merkel cell carcinomas), 150 endocrine tumors (thyroid, parathyroid, adrenocortical, and pheochromocytomas/paragangliomas), 279 adenocarcinomas, and 88 squamous cell carcinomas of various organs, including those of the lungs and others. In addition, normal tissues from various organs were studied. OTP nuclear expression was seen in 80% of lung carcinoid tumors. Among other tumors, 4 small-cell carcinomas showed focal expression (2 pulmonary and 2 bladder), but all other tumors were completely negative. Overall, the sensitivity and specificity of OTP were 80.2% and 99.4%, respectively. All TTF1-positive lung carcinoid tumors were diffusely positive for OTP, but none of the OTP-negative carcinoid tumors was positive for TTF1. OTP expression was not seen in any normal tissues/organs. OTP was also negative in neuroendocrine cells of the normal bronchus/bronchiole. However, OTP was strongly expressed in neuroendocrine hyperplasia, including reactive and preneoplastic hyperplasia. Our results suggest that OTP may serve as a useful diagnostic marker for lung carcinoid tumors.