Résumé : Graft-versus-host disease (GvHD) assessment has been shown to be a challenge for healthcare professionals, leading to the development of the eGVHD App (www.uzleuven.be/egvhd). In this study, we formally evaluated the accuracy of using the App compared to traditional assessment methods to assess GvHD. Our national multicenter randomized controlled trial involved seven Belgian transplantation centers and 78 healthcare professionals selected using a 2-stage convenience sampling approach between January and April 2017. Using a 1:1 randomization stratified by profession, healthcare professionals were assigned to use either the App ("APP") or their usual GvHD assessment aids ("No APP") to assess the diagnosis and severity score of 10 expert-validated clinical vignettes. Our main outcome measure was the difference in accuracy for GvHD severity scoring between both groups. The odds of being correct were 6.14 (95%CI: 2.83-13.34) and 6.29 (95%CI: 4.32-9.15) times higher in favor of the "APP" group for diagnosis and scoring, respectively (P<0.001). App-assisted GvHD severity scoring was significantly superior for both acute and chronic GvHD, with an Odds Ratio of 17.89 and 4.34 respectively (P<0.001) and showed a significantly increased inter-observer agreement compared to standard practice. Despite a mean increase of 24 minutes (95%CI: 20.45-26.97) in the time needed to score the whole GvHD test package in the "APP" group (P<0.001), usability feedback was positive. The eGVHD App shows superior GvHD assessment accuracy compared to standard practice and has the potential to improve the quality of outcome data registration in allogeneic stem cell transplantation.