Differential diagnostic problems between gastric carcinomas and precancerous lesions with severe dysplasia have become more perceptible with the increasing number of resected early carcinomas. Although such problems come up for all macroscopic and histologic types of gastric cancer they are particularly marked between early carcinomas of the elevated type and adenomatous polyps. Elevated early carcinomas are usually highly differentiated adenocarcinomas with a morphology which often reminds of of adenomas. But sometimes the carcinomas also demonstrate convincing signs of being developed from adenomas. The criterion of distinction between intramucosal carcinomas and adenomas is invasion through the basal membrane, often difficult to evaluate. The morphological relation between elevated early gastric carcinomas and adenomas and the criterion of distinction between them were studied in 20 early gastric carcinomas of the Japanese types I and IIa, 6 intramucosal and 14 submucosal all highly differentiated adenocarcinomas, and in 42 polyps, of which 5 were of the adenomatous type. All lesions were taken from resection specimens. Among the carcinomas 5 demonstrated convincing signs of being malignant transformed adenomas. In addition, 6 carcinomas had a morphology which more or less reminded of adenomas, but their genetic origin was more uncertain. Nine carcinomas revealed no sign of an adenomatous origin. Among the 5 polyps diagnosed as adenomas 2 revealed an extraordinary degree of severe dysplasia which caused uncertainty on the benign diagnosis. The rest of the polyps were without dysplasia. The significance of invasion through the basal membrane as an indispensable factor of distinction between adenoma and carcinoma in the stomach is discussed. It is concluded that the degree of dysplasia can be so severe and the invasion so difficult to evaluate that the classification of some few tumours depends on the subjectivity of the single pathologist. Four of the tumours, 2 adenomas and 2 intramucosal carcinomas, having a remarkable macroscopic appearance like a large mucosal fold are especially mentioned. Their relation to gastric mucosal prolaps is discussed. Furthermore, a tumour apparently demonstrating only a moderate degree of dysplasia, but even so setting up metastases is mentioned in detail.