A 69-year-old woman with gastritis was prescribed a 1-week triple regimen therapy of Amoxicillin, Clarithromycin, and Lansoprazole to eradicate Helicobacter pylori (H. pylori) starting on March 7, 2005. H. pylori was detected on the gastric mucosa by the urease test. Twenty days after eradication therapy, she began to suffer from profuse watery diarrhea. Colonoscopy on April 12 showed multiple pseudomembranes in the cecum and the transverse colon, leading to a diagnosis of pseudomembranous colitis. Because she had not taken Vancomycin (VCM) (500 mg/day) as directed, she had a relapse of watery diarrhea and was admitted on April 30. A stool test for Clostridium difficile (CD) toxin was positive, although colonoscopy showed only a few aphthoid erosions in the cecum and the transverse colon on May 6. She was treated with oral VCM (2000 mg/day) from May 6, and diarrhea disappeared by May 11. The stool test for CD toxin was negative, so VCM was discontinued. Care must thus be taken in H. pylori eradication to ensure that the triple regimen therapy does not lead to pseudomembranous colitis.