Previous studies suggest that gut microbiome disruption induced by chemotherapy, dietary deficiencies, and/or antibiotics are associated with increased incidence of acute graft-versus-host disease (aGVHD) following hematopoietic stem cell transplantation (HSCT). In a murine model of antibiotic-induced gut microbiome disruption, Holmes and colleagues show that oral administration of galactooligosaccharides (GOS) as a prebiotic attenuates lethal aGVHD, highlighting the crosstalk between diet and gut microbiota. Their data encourage clinical trials of GOS prebiotic diets during HSCT.