The characteristics and histogenesis of gastric-type adenocarcinomas were studied for endoscopically removed hyperplastic polyps and intramucosal cancers found in surgically resected stomachs (m-cancers). Among 421 hyperplastic polyps, 14 differentiated-type carcinomas were found (HP-cancers). Eleven (78.6%) of these lesions were gastric-type adenocarcinomas. Out of 65 m-cancers, 22 were undifferentiated-type carcinomas and 43 were differentiated-type carcinomas, the latter being classified into 10 gastric-type adenocarcinomas (23.2%) and 13 intestinal-type adenocarcinomas: the remaining 20 were of mixed gastric and intestinal type. The mean age of the gastric-type adenocarcinoma patients did not differ from that of patients with other differentiated-type carcinomas. No appreciable signs of intestinal metaplasia wee noted in HP-cancer polyps. In m-cancers, the degree of intestinal metaplasia of the surrounding mucosa of gastric-type adenocarcinomas tended to be lower than in the other differentiated-type carcinomas, indicating a weak relationship between the histogenesis of gastric-type adenocarcinomas and intestinal metaplasia. Studies by PCNA (proliferating cell nuclear antigen) immunohistochemistry, showed that in over half of the gastric-type adenocarcinomas cases PCNA-positive cells tended to be localized within tumor tissues. In addition, point mutations of the c-Ki-ras gene were detected in 1 gastric-type adenocarcinoma and 2 intestinal-type adenocarcinomas, suggesting the occurrence of a common genetic abnormality.