The results of gastric secretory studies in 192 cases of recurrent ulcer after surgery for duodenal ulcer were analyzed and compared with the secretory data collected in a control group of 74 duodenal ulcer patients who had undergone various forms of gastric surgery, but who did not develop a recurrent ulcer (controls). The patients studied comprised 46 cases of recurrent ulcer after partial gastrectomy, 10 cases of recurrent ulcer after partial gastrectomy and bilateral truncal vagotomy, 56 cases of recurrent ulcer after truncal vagotomy and drainage, 52 cases of recurrent ulcer after highly selective vagotomy, and finally 28 cases in which the recurrent ulcer led to the diagnosis of the Zollinger-Ellison syndrome. The entire study was based upon an analysis of the basal acid output, the response to maximal stimulation by pentagastrin or by histalog and by insulin in the case of previous vagotomy, and finally on an assessment of basal serum gastrin. The analysis has suggested minimal secretory levels with discriminative values useful for the postoperative diagnosis of recurrent ulcer and for an assessment of the completeness of vagotomy (ratio PAO Insulin/PAO pentagastrin or histalog). Moreover, an analysis of various elements of the sequential basal pentagastrin-insulin test permitted us to approach the pathophysiological mechanism responsible for ulcer recurrence, and to identify suitable criteria for selection of the best treatment.