The results of hemigastrectomy combined with vagotomy are superior to those of subtotal gastrectomy from every viewpoint, in our experience. These two procedures have been compared on the basis of operative mortality, clinical results, frequency of recurrent ulceration and the physiologic effect on the acidity and effective peptic activity of the gastric contents. We believe that hemigastrectomy with vagotomy is a very effective operation and we employ it as the procedure of choice in the treatment of duodenal ulcer. With regard to more conservative procedures, our own experience with them does not permit an accurate comparison at this time. We will discuss this matter at a later date. So far as one can judge from a comprehensive survey of the literature, lesser procedures such as vagotomy combined with gastroenterostomy or pyloroplasty are reported to have an operative mortality which is lower by about 1 per cent [2,12]. On the other hand, as we have indicated, if one has to deal with significant numbers of patients who are poor risks, the operative mortality for the most conservative procedures may be as high as 5 per cent [1]. They certainly should be used when closure of the duodenal stump is hazardous. Whether the operative mortality in other patients who are poor risks will be lowered or not is open to some question. We find no evidence in the literature [2,12] or in our own limited experience that the clinical results of lesser procedures are superior to those of hemigastrectomy with vagotomy. With regard to recurrent ulceration, there is no doubt that the results of lesser procedures are inferior. On the basis of the physiologic effect of these operations, on the acidity and peptic activity of the gastric contents, we suspect that the eventual outcome of these operations will be disappointing and that the morbidity and mortality related to recurrent ulceration will more than counterbalance unfavorably the questionable advantage of these procedures in terms of initial operative mortality. Insofar as we can judge from the literature, the recurrence rate for vagotomy with gastroenterostomy or pyloroplasty among patients followed for a period comparable to those for patients discussed in this report varies from 5 to 10 per cent. The experience of Colp [1] seems pertinent in this regard. In a 1 to 9 year follow-up period, recurrent ulcers developed in 0.5 per cent of his patients following subtotal gastrectomy combined with vagotomy, in 7.7 per cent of patients following subtotal gastrectomy, in 10 per cent of his patients following gastroenterostomy combined with vagotomy and in 15 per cent of his patients having gastroenterostomy alone. With regard to the physiologic aspects of this problem, which we believe will eventually prove to be most important, our data permit certain statements regarding the question of which patients will develop recurrent ulceration following various operations. We think that this is an important matter since, so far as we know, the purpose of an operation for duodenal ulcer is to cure the ulcer and prevent its recurrence. On the basis of our physiologic evidence to date, we can say the following. (1) No patient, whose gastric contents are achlorhydric under basal conditions of study and in response to stimulation by peptomized beef broth and insulin (pH 3.5 or more), has had a recurrent ulcer. (2) All patients with recurrent ulceration have free hydrochloric acid in their gastric contents under one or all of the above conditions of study (pH below 3.5). (3) Patients with free hydrochloric acid in their gastric contents following stimulation by insulin, who appear to be protected against recurrent ulceration, are contained within the following categories: (a) The quantity of free hydrochloric acid expressed in milliequivalents per liter is in the range 0.03 to 0.3. (b) The acidity of the gastric contents is in the range pH 3.4 to 2.7. (c) The effective peptic activity of the gastric contents is below 417 units per hour. Data concerning the acidity and effective peptic activity of the gastric contents permit the evaluation of parietal and chief cell activity separately and would appear to open new areas of the peptic ulcer problem to exploration.