Background: Gastrointestinal graft-versus-host disease (GvHD) is a potentially life-threatening complication after allogeneic stem cell transplantation (SCT). Since therapeutic options are still limited, a prophylactic approach seems to be warranted. Methods: In this randomised, double-blind-phase III trial, we evaluated the efficacy of budesonide in the prophylaxis of acute intestinal GvHD after SCT. The trial was registered at https://clinicaltrials.gov, number NCT00180089. Patients were randomly assigned to receive either 3 mg capsule three times daily oral budesonide or placebo. Budesonide was applied as a capsule with pH-modified release in the terminal ileum. Study medication was administered through day 56, follow-up continued until 12 months after transplantation. If any clinical signs of acute intestinal GvHD appeared, an ileocolonoscopy with biopsy specimens was performed. Results: The crude incidence of histological or clinical stage 3-4 acute intestinal GvHD until day 100 observed in 91 (n =48 budesonide, n =43 placebo) evaluable patients was 12.5% (95% CI 3-22%) under treatment with budesonide and 14% (95% CI 4-25%) under placebo (p = 0.888). Histologic and clinical stage 3-4 intestinal GvHD after 12 months occurred in 17% (95% CI 6-28%) of patients in the budesonide group and 19% (CI 7-32%) in the placebo group (p = 0.853). Although budesonide was tolerated well, we observed a trend towards a higher rate of infectious complications in the study group (47.9% versus 30.2%, p = 0.085). The cumulative incidences at 12 months of intestinal GvHD stage >2 with death as a competing event (budesonide 20.8% vs. placebo 32.6%, p = 0.250) and the cumulative incidence of relapse (budesonide 20.8% vs. placebo 16.3%, p = 0.547) and non-relapse mortality (budesonide 28% (95% CI 15-41%) vs. placebo 30% (95% CI 15-44%), showed no significant difference within the two groups (p = 0.911). The trial closed after 94 patients were enrolled because of slow accrual. Within the limits of the final sample size, we were unable to show any benefit for the addition of budesonide to standard GvHD prophylaxis. Conclusions: Budesonide did not decrease the occurrence of intestinal GvHD in this trial. These results imply most likely that prophylactic administration of budenoside with pH-modified release in the terminal ileum is not effective.