Malignant degeneration is the most serious complication of gastric ulcer. Its recognition is difficult both in the early stage and in advanced cases in which only the evidence of a previous ulcer-cavity, and the radiating folds of the mucous membrane indicate progressive development of carcinoma from an original ulcer.It is impossible to say how often gastric ulcer becomes malignant; one can only state the frequency of ulcer-carcinoma, found in gastric resections.One hundred and forty-one personal cases of ulcer-carcinoma are recorded, and are divided into three groups.GROUP I: 41 which were diagnosed clinically and at operation as cases of ulcer, but in which histological examination showed incipient cancer.GROUP II: 55 diagnosed clinically as cases of ulcer, but in which a diagnosis of ulcer-carcinoma was made during operation and afterwards histologically confirmed.GROUP III: 45 diagnosed both clinically and macroscopically (from the typical folding of the mucous membrane) as cases of ulcer-cancer, in which the cancer had entirely overgrown the ulcer.Therefore in the series of 532 resections for gastric ulcer the frequency of ulcer-carcinoma was 20.9%, or 15.2% if the third group is omitted.In a series of 718 resections for gastric cancer, the frequency of ulcer-carcinoma was 19.6% (or 14.2% if the third group is omitted).The mortality in simple two-third resection of the stomach is low (four deaths in 99 cases = 4%). When the pancreas, liver, colon, or oesophagus, is involved, the resection mortality is high (14 deaths in 42 cases = 33.3%), but even in these cases the operation is justifiable because permanent cures were achieved in a number of cases.The prognosis in cases of ulcer-cancer is very grave. In many cases, judging from the author's own experience, patients suffering from incipient ulcer-cancer-only histologically diagnosed as cancer-die from liver metastases, in spite of radical resection. It will thus be seen that the end-results of resection for ulcer-carcinoma are actually worse than those of resection for primary carcinoma.A. Ulcer-cancer: In Group I, 35 cases were operated on before 1933, and in 18 of these (51.4%) the patients have been free from symptoms for more than five years; in Group II, 27 cases were operated on before 1933, and in four of these (14.8%) the patients are still symptom-free. In Group III, out of 37 cases operated on, only two patients (5.4%) have been symptom-free for the same period.B. Primary cancer: Out of 260 cases of resection for primary cancer before 1933, 77 patients (29.6%) are permanently cured.If the ulcer-cancer is so far advanced that the diagnosis can be made clinically, or during operation, the prognosis is extremely bad (permanent cures having been only 9.3% in the series).In cases of gastric ulcer the best plan is to carry out resection before malignant degeneration begins. The result would then be that not merely 51% but at least 90% of the patients would be alive and well after five years.