Saccharomyces boulardii is a probiotic yeast that has been shown to have beneficial effects on the intestinal epithelial barrier and digestive immune system. There is preliminary evidence that S boulardii could be used to treat patients with Crohn's disease (CD). We performed a randomized, placebo-controlled trial to evaluate the effects of S boulardii in patients with CD who underwent remission during therapy with steroids or aminosalicylates. We performed a prospective study of 165 patients who achieved remission after treatment with steroids or salicylates; they were randomly assigned to groups given S boulardii (1 g/day) or placebo for 52 weeks. The primary end point was the percentage of patients in remission at week 52. Time to relapse, Crohn's disease activity index scores, and changes in parameters of inflammation were secondary end points. CD relapsed in 80 patients, 38 in the S boulardii group (47.5%) and 42 in the placebo group (53.2%, a nonsignificant difference). The median time to relapse did not differ significantly between patients given S boulardii (40.7 weeks) vs placebo (39.0 weeks). There were no significant differences between groups in mean Crohn's disease activity index scores or erythrocyte sedimentation rates or in median levels of C-reactive protein. In a post hoc analysis, nonsmokers given S boulardii were less likely to experience a relapse of CD than nonsmokers given placebo, but this finding requires confirmation. Although the probiotic yeast S boulardii is safe and well tolerated, it does not appear to have any beneficial effects for patients with CD in remission after steroid or salicylate therapies. Copyright © 2013 AGA Institute. Published by Elsevier Inc. All rights reserved.