Eighteen cases of primary mucinous early gastric cancer (EGC) were studied with regard to histogenetic implications. This type of carcinoma was found in 10.7% of 168 cases of EGC and was removed surgically. In 11 patients, mucinous tumor was mainly found in the submucosal layer of the stomach. These findings are in keeping with the hypothesis that mucinous adenocarcinoma develops during progression of an ordinary adenocarcinoma. In two cases of well-differentiated mucinous intramucosal EGC, mucinous tumor was associated with a villous adenoma, as is the case in intestinal-type adenocarcinoma. In five cases of poorly differentiated mucinous carcinoma, tumor was seen at the middle level of the mucosa, with normal foveolae in the upper mucosa and normal specialized antral and body type glands in the lower part. There were no associated areas of dysplasia or intestinal metaplasia. These histologic findings suggest that a subset of mucinous EGC develops from the proliferative zone (the neck region) of nonmetaplastic glands, as reported in diffuse carcinoma of the stomach. These results suggest a histogenetic heterogeneity in the entity of the mucinous carcinoma. Well- and poorly differentiated mucinous intramucosal EGCs show a histogenesis similar to that of gastric carcinoma of the intestinal and diffuse types, respectively.