Graft-versus-host disease 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 graft-versus-host disease. Our national multicenter randomized controlled trial involved seven Belgian transplantation centers and 78 healthcare professionals selected using a two-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 graft-versus-host disease assessment aids ("No APP") to assess the diagnosis and severity score of ten expert-validated clinical vignettes. Our main outcome measure was the difference in accuracy for graft-versus-host disease 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 graft-versus-host disease severity scoring was significantly superior for both acute and chronic graft-versus-host disease, 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 time needed to score the whole graft-versus-host disease test package in the"APP" group (p<0.001), usability feedback was positive. The eGVHD App showed superior graft-versus-host disease assessment accuracy compared to standard practice and has the potential to improve the quality of outcome data registration in allogeneic stem cell transplantation.