A clinicopathologic study was done on 21 cases of superficial flat-type early gastric carcinoma (IIb type EGC). In one case there was the two IIb type EGC. Nine patients had no symptoms, whereas the other 12 had either epigastralgia, hematemesis, or anorexia. The preoperative diagnosis was accurate in 15 patients; eight were demonstrated by barium study, and 13 by endoscopy. The suspicious finding of IIb type EGC was either the disappearance or irregularity of the areae gastricae by barium study and a mucosal color change by endoscopy. Well-differentiated or moderately differentiated adenocarcinomas showed a slight redness of the affected mucosa whereas the poorly differentiated adenocarcinomas were pale in color. Histologically, many well-differentiated or moderately differentiated adenocarcinomas occupied the entire thickness of the mucosal layer whereas most of the poorly differentiated adenocarcinomas spread horizontally with preservation of non-cancerous glands and foveolae. The growth pattern was super type in ten lesions and small mucosal type in 12 and no pen-type growth was seen. Concerning the cell nuclear DNA ploidy pattern, 21 showed a low ploidy pattern and only one had a high ploidy pattern. The IIb type EGC seemed to have a less malignant potential from the viewpoint of growth pattern and DNA ploidy pattern. Care must be taken at the proximal line of excision of the tumor so as not to leave behind residual carcinoma cells.