About 5--10% of the gastric carcinomas develop from a gastric ulcer (ulcerocancer). This process, however, is, referred to gastric ulcers, too rare to indicate a prophylactic resection of the stomach on account of the danger of the development of cancer. Careful diagnostics and following control with X-ray examination and gastroscopy are, however, necessary in every gastric ulcer. The retrogression of the complaints neither proves healing of the ulcer nor benignity of the ulcer. Compared with the ulcerocarcinoma the differential diagnosis of the gastric ulcer is easy when locally advanced, patelliform carcinomas are in question. It is difficult in small carcinomas and especially in the carcinoma of the mucous membrane of type III. Individual radiologic and endoscopic signs and especially their combination are useful for the differentiation, but they may be misleading. A carcinomatous ulcer may radiologically and endoscopically look like a peptic ulcer and may apparently also fully heal. Therefore, a certain differential diagnosis is only possible with the help of an aimed biopsy of the stomach or the cytodiagnostics. The two methods serve further propagation. They should be used in every case, when an apparantly benign ulcer does not reveal a clear retrogression under effective treatment with carbenoxolone within four weeks. The examination of the gastric juice may be used for the differential diagnosis.