We investigated the immunohistochemical characteristics of 13 cases of odontogenic carcinoma, including 3 cases of ameloblastic carcinoma, 9 cases of primary intraosseous squamous cell carcinoma (PIOSCC), and 1 case of ghost cell odontogenic carcinoma (GCOC), to elucidate the origin and determine diagnostic markers of odontogenic carcinoma. Thirty cases of gingival squamous cell carcinoma (gSCC) were included for comparison. All ameloblastic carcinomas and GCOC cases, as well as 6 of 9 PIOSCC cases, exhibited immunoreactivity for CK19, whereas only 3 of 30 gSCC cases were positive for CK19. Two cases of ameloblastic carcinoma and only 1 case of PIOSCC expressed CD56; however, CD56 was not expressed in any gSCC cases. All odontogenic carcinoma cases except a single GCOC case were negative for calretinin. These findings indicate that immunoreactivity to CK19 is useful to identify odontogenic origin of the odontogenic carcinomas including PIOSCC, and it may also be useful for distinguishing PIOSCC from gSCC.