The purpose of this study was to retrospectively investigate the effect of the severity of acute graft-versus-host disease (acute GVHD) on physical function after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Methods 76 patients were included as subjects of this study. Severity of acute GVHD was classified according to the method defined by Grucksberg. To evaluate physical function, the knee extensor strength and six-minute walk distance (6MWD) were performed. Results Among these patients, 54% developed acute GVHD; of these 32%, 54%, and 15% of patients had Grade I, Grade II, and Grade III-IV GVHD, respectively. In the Grade I-II groups, mild acute GVHD following allo-HSCT, resulted in a gradual decline in physical function, which improved at discharge. However, in cases of severe acute GVHD, physical function deteriorated, implementation of rehabilitation became difficult, and the decline in physical function persisted even at discharge. Conclusion These results indicate that severe acute GVHD negatively affects physical function leading to longer hospital days because of inadequate rehabilitation interventions.