Crohn's disease (CD) is characterized by a reduction in mucosal integrity that permits antigen penetration into the intestinal tissue. The administration of probiotics has been suggested to improve the barrier function of the mucosa. The objective of this study was to evaluate the influence of Saccharomyces boulardii on the intestinal permeability in CD. Thirty-four patients were randomized according to the Vienna classification for treatment with either placebo or Saccharomyces boulardii. Baseline medications (mesalamine, azathioprine, prednisone, metronidazole and/or thalidomide) were maintained. Intestinal permeability (lactulose/mannitol ratio) was evaluated immediately before the beginning of treatment and at the end of the first and third treatment month. Fifteen healthy volunteers were also submitted for the intestinal permeability test. In volunteers, the lactulose/mannitol ratio was 0.005+/-0.0037, whereas this value was 0.021+/-0.01 in patients with CD (p=0.001). In the placebo group, there was an increase in lactulose/mannitol ratio by 0.004+/-0.010 (p=0.12) at the end of the third month. In the S. boulardii group, there was an improvement in intestinal permeability, with a decrease in the lactulose/mannitol ratio by 0.008+/-0.006 (p=0.0005) in the same period. Patients with CD in remission present alterations in the integrity of the intestinal mucosal barrier according to lactulose/mannitol ratio. S. boulardii added to baseline therapy improved intestinal permeability in these patients, even though complete normalization was not achieved.