The authors report two cases of hypereosinophilia as the major presenting sign of acute graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT). Tissue biopsies of the skin, salivary gland, gut, and liver showed evidence of acute GVHD (aGVHD). In one case, further investigations have been performed. Elevated levels of interleukin (IL)-5 and soluble IL-2 receptor were found in the blood, and skin biopsy specimens demonstrated high levels of IL-5 messenger ribonucleic acid (mRNA). In contrast, skin biopsy specimens from other patients with aGVHD but without eosinophilia were negative for IL-5 mRNA. The authors also demonstrated the presence of IL-4 and interferon(IFN)-gamma mRNA within the same skin biopsy specimen, suggesting that this case of aGVHD was mediated by both Th1 and Th2 cell type. These two patients were treated by glucocorticoids with resolution of the hypereosinophilia and the symptoms of GVHD. The authors briefly discuss the possible mechanisms of this hypereosinophilia with respect to aGVHD.