Abstract: Idiopathic interstitial pneumonia (IIP) can occur after stem cell transplantation, but the aetiology is unknown. Based on the association between angiitis syndrome and Helicobacter pylori infection, we identified possible risk factors common to these two conditions. Among 83 patients who underwent stem cell transplantation, four developed IIP. We elucidated various parameters and clinical features in four patients with IIP and 79 patients without, after allogeneic stem cell transplantation. In all four patients, (1) the conditioning regimen induced total body irradiation, (2) serological reactivation of cytomegalovirus and/or human herpesvirus-6 preceded the onset of IIP, (3) their human leucocyte antigen types were among those suspected to increase susceptibility to angiitis syndrome, (4) serum anti-H. pylori antibody was positive before conditioning and remained positive throughout the post-transplantation course, (5) inflammatory cytokines (interleukin-6, 8 and 12) were increased during the period of leucocyte recovery after transplantation and (6) the levels of intercellular adhesion molecule-1, thrombomodulin and plasminogen activator inhibitor-1 were increased at the onset of IIP. These findings suggest the possibility that angiitis syndrome and H. pylori infection are involved in the pathogenesis of post-transplantation IIP.