Successful bowel preparation is essential to an adequate performance of colonoscopy. Polyethylene glycol (PEG) with electrolyte solutions induces diarrhea with depletion of substrates fermentable by hydrogen (H2)-producing colonic microbiota. Inulin has recently been suggested as a prebiotic substrate for the H2 breath test because it is resistant to intestinal hydrolysis and is fermented mostly by the colonic bacteria. This study aimed to assess time-dependent changes in H2 breath levels in order to predict the colonic preparation of patients scheduled for colonoscopy with or without oral supplementation of inulin. METHODS: In this prospective nonrandomized trial, 127 subjects drank 4 l of PEG 280-mg solution as bowel preparation for colonoscopy. A subgroup of 31 patients also ingested inulin (10 g in 200 ml of water) at breakfast as an additional substrate to increase colonic H2 production. Measurements of H2 breath levels were performed immediately before and after colonic preparation. As the main outcome measure, the quality of the colonic preparation was scored as excellent to fair (i.e., clean bowel allowing successful pan-colonoscopy, including the terminal ileum) or poor (incomplete colonoscopy due to fecal debris). RESULTS: The H2 breath levels decreased from 11.0 ± 1.8 ppm before PEG to 1.8 ± 0.3 ppm after PEG (n = 18; P