AIM: To investigate the effect of probiotic bacterium, Clostridium butyricum MIYAIRI 588 strain (CBM) on the changes of the fecal flora in Helicobacter pylori (H pylori) treatment. METHODS: Thirty-five patients with gastric or duodenal ulcers positive for H pylori were randomized either to 1 wk amoxicillin, clarithromycin, lansoprazole (Group 1) or to the same regimen supplemented with CBM 7 d ahead of the triple therapy (Group 2). Stool samples were collected before and 2, 4, 7, 15, and 22 d after the starting eradication therapy, and were examined intestinal flora. Patients were required to keep a diary record of their condition. RESULTS: Obligate anaerobes decreased significantly on d 2, 4, 8 and 15 in Group 1. On the other hand, they did not decrease significantly in Group 2. The Escherichia coli was dominant bacterium in Enterobacteriaceae, but that was replaced by other species such as Klebsiella and Enterobacter after eradication in Group 1. The change was suppressed in Group 2. Abdominal symptoms were less frequent in Group 2 than in Group 1. CONCLUSION: The combined use of CBM reduced the changes in the intestinal flora and decreased the incidence of gastrointestinal side effects. AIM: To investigate the effect of probiotic bacterium, Clostridium butyricum MIYAIRI 588 strain (CBM) on the changes of the fecal flora in Helicobacter pylori (H pylori) treatment. METHODS: Thirty-five patients with gastric or duodenal ulcers positive for H pylori were randomized to either wk amoxicillin, clarithromycin, lansoprazole (Group 1) or to the same regimen supplemented with CBM 7 d ahead of the triple therapy (Group 2). Stool samples were collected before and 2, 4, 7, 15, and 22 Patients were required to keep a diary record of their condition. RESULTS: Obligate anaerobes decreased significantly on d 2, 4, 8 and 15 in Group 1. On the other hand, they did not decrease significantly in Group 2. The Escherichia coli was dominant bacterium in Enterobacteriaceae, but that was replaced by other species such as Klebsiella and Enterobacter after eradication in Group 1. The change was suppressed in Group 2. Abdominal symptoms were less frequent in Group 2 than in Group 1. CONCLUSION: The combined use of CBM reduced the changes in the intestinal flora and decreased the incidence of gastrointestinal side effects.