Individuals transplanted with hematopoietic stem cells from donors carrying a long homozygous (GT)<sub>n</sub> repeat polymorphism (L/L) in the <i>HMOX1</i> promoter, which is associated with lower HO-1 expression, were at higher risk of developing severe acute GVHD as compared to donors carrying a short (GT)<sub>n</sub> repeat (S/L or S/S) polymorphism associated with higher HO-1 expression. This evidence concerns the gene HMOX1 and acute graft versus host disease.