Twenty-eight cases of primary adenosquamous carcinoma (ASC) of the stomach were studied clinicopathologically. These cases were classified into two types, 16 with differentiated type adenocarcinomatous component (DAC) and 12 with undifferentiated type adenocarcinomatous component (UAC), according to the degree of glandular formation of the adenocarcinomatous elements. A large number with adenocarcinomas, including 131 with differentiated type and 133 with undifferentiated type, were studied as controls. As a consequence, with respect to biologic behavior, ASC with DAC was similar to the differentiated type adenocarcinoma, and ASC with UAC to the undifferentiated type adenocarcinoma. Accordingly, the behavioristic feature of ASC seemed to be governed by the adenocarcinomatous component. Histologically, a close relationship between neoplastic adenomatous and squamous components was evident in the intermingling areas, thereby suggesting a transition of both elements. In addition, a mucoepidermoid pattern was occasionally detected in the squamous component. Judging from the biologic behavior and histologic findings, the majority of ASC probably derives from the squamous metaplasia in an adenocarcinoma. The prognosis of ASC was less favorable than that of adenocarcinoma because of the more extensive tumor depth and higher frequencies of lymphatic and vascular permeations of the carcinoma cells.