Gastrointestinal bleeding caused by peptic ulcer disease is one of the serious complications of living-related liver transplantation (LRLT). The aim of this study was to clarify the factors involved in peptic ulcer formation in adult LRLT recipients. Forty consecutive adult LRLT recipients without a history of peptic ulcer disease were studied. Twenty-five patients (62.5%) tested positive for Helicobacter pylori. After LRLT, duodenal ulcer (DU) developed in six patients, and all of them tested positive for H. pylori. In contrast, none of the H. pylori-negative patients developed DU. Preoperative serum gastrin levels in patients with DU were significantly higher than in those without DU, irrespective of H. pylori infection. Preoperative pepsinogen I levels in patients with DU were significantly higher than in those without DU with H. pylori infection. These data suggest involvement of H. pylori infection in the development of DU after LRLT. Eradication of H. pylori may prevent the development of DU after LRLT particularly in patients with hypergastrinemia and high serum pepsinogen I.