In a prospective study we investigated the clinical value of ambulatory 24-hour pH monitoring in the stomach after Billroth-II-resection. Recorded data of 10 patients with and 11 patients without ulcer recurrence after gastric resection were compared. The pH medians of the record periods "total time", "supine", "upright", "fasting" and "postprandial" were evaluated. Further the cumulative recording time of pH levels pH < 1, pH < 2, etc. was determined and compared in both groups. A pH median less than 2.8 "supine" indicated a postoperative ulcer recurrence with a sensitivity of 90% and specificity of 81%. A pH median less than 2.6 "fasting" predicted an ulcer relapse with a sensitivity of 90% and specificity of 72% (p < 0.001). In patients with ulcer recurrence pH was < 4 in more than 30% of total recording rime, pH < 5 in more than 45% and pH < 6 in more than 75% (sensitivity 90%, specificity 100%). According to these findings 24-hour pH monitoring seems to be useful for the prognostic assessment of the further course of ulcer disease after B II-surgery of the stomach. Thus pH monitoring results may be helpful in defining patient groups which may be candidates for acid reducing therapy to prevent ulcer recurrence.