In a randomized, single-center, double-blind, placebo-controlled pilot study, 20 patients with established Crohn's disease suffering from diarrhea and moderate complaints as measured by the BEST Index, were treated with the yeast preparation Saccharomyces boulardii (S.b.) in a dosage of 250 mg t.i.d., initially for two weeks in addition to the basic treatment. A reduction in the frequency of bowel movements (5.0 +/- 1.4 vs. 4.1 +/- 2.3 evacuations/day, p < 0.01) and in the BEST Index (193 +/- 32 vs. 168 +/- 59, p < 0.05) as compared to baseline was registered. After this initial phase, the patients were allocated in randomized order to the control group (n = 7) receiving placebo, or to the verum group (n = 10) receiving S.b.(250 mg t.i.d.) for 7 weeks, while the basic treatment was maintained. The group treated with S.b. showed a significant reduction in the frequency of bowel movements in the tenth week, to 3.3 +/- 1.2 evacuations per day, and in the BEST Index, to 107 +/- 85. In the control group taking placebo, however, this effect was not observed. By contrast, the frequency of bowel movements and the BEST Index rose again in the tenth week until reaching initial values (4.6 +/- 1.9 evacuations daily and 180 +/- 61, respectively). No adverse drug events were observed. In order to confirm these positive effects of S.b. in patients with Crohn's disease, further controlled multicenter trials in a larger patient population should be performed.