The aim of the present study was to evaluate the role of gastric acid secretory tests in predicting ulcer recurrence in a consecutive series of patients before and up to 5 years after parietal cell vagotomy (PCV). Included in the study are 405 patients with prepyloric or duodenal ulcer disease (57 recurrences). There were no differences in acid secretion in the group of patients with pyloric or prepyloric ulcer disease, with or without recurrences. Thus pre- and post-operative acid secretory tests are of no value in predicting the outcome of PCV in this group of patients. In the duodenal ulcer group both postoperative basal acid output and peak acid output after pentagastrin and after insulin were higher in patients with recurrences. The two best discriminatory values were the ratio between the preoperative and the 6-week value of basal acid output and the 6-week value of peak acid output after insulin. The results of the study suggests that the ratio of the basal output before surgery and 6 weeks after surgery is enough for judging the acid reduction after PCV and that only in the case of a recurrence should an acid stimulation test be used.