Relapse of duodenal ulcers was observed endoscopically after Helicobacter pylori eradication therapy for gastric ulcer patients in 2 of 32 successful cases. One patient, a 40-year-old woman, received dual therapy with lansoprazole 60mg and amoxicillin 1000mg for 2 weeks because of an intractable, easily-relapsing gastric ulcer accompanied by duodenal ulcer scars that had not relapsed for 5 years. The H. pylori status was assessed by a rapid urease test, light microscopy, culture, and anti-H. pylori antibody. At 24 months after the cure of H. pylori she had upper abdominal pain and showed relapse not of the gastric ulcer but of the duodenal ulcer. The H. pylori status remained negative. The other patient, a 44-year-old man, showed an active gastric ulcer and duodenal ulcer scars at the first endoscopy. He received the same regimen as described above. Ten weeks after completion of the eradication therapy, endoscopy showed healing of the gastric ulcer and relapse of the duodenal ulcer despite successful eradication. These two cases suggest that H. pylori eradication modifies the pathophysiological condition of gastric acid secretion and facilitates relapse of duodenal ulcers.