The aim of the present study was to determine the differences in gastric acidity between duodenal ulcer patients with and without recurrent ulceration after elective proximal gastric vagotomy (PGV). The pre- and postoperative results of 24-hour intragastric pH monitoring of 10 patients with ulcer relapse were compared to the data from a control group of 42 patients without recurrence. Patients with ulcer recurrence had a significantly (p < 0.05) lower median pH and a significantly (p < 0.05) higher median percentage of pH recordings of ≤ 2 during the postoperative sleep phase than patients without ulcer relapse. High nocturnal acidity seems to be a predisposing factor for the recurrence of ulcers after PGV. A risk group for ulcer recurrence after PGV is defined by a postoperative median pH of ≤ 2.0 and a median percentage of pH readings of ≤ 2 of more than 70% during the sleep phase. The sensitivity and specificity of postoperative intragastric pH monitoring for identification of this at-risk group was 80 and 91 %, respect