This study was undertaken to identify the factors influencing pulmonary function in patients who underwent hematopoietic stem cell transplantation (HCT). Pulmonary function tests were evaluated before and after HCT in 51 adult patients who underwent HCT between 1993 and 1998. The patients with hematologic malignancies were given total body irradiation. Graft-versus-host disease (GVHD) prophylaxis consisted of cyclosporine A and short-term methotrexate. Six patients suffered from acute GVHD above grade II and 27 patients suffered from chronic GVHD. The post-transplant % diffusing capacity (%DLco) 100 days after HCT was significantly lower than pretransplant values (82 +/- 21% versus 71 +/- 15%, p < 0.01). The %DLco at 100 days was significantly lower in patients with chronic GVHD than in patients without chronic GVHD (66 +/- 16% versus 77 +/- 9%, p < 0.05). These findings suggested chronic GVHD is related to the decreased %DLco values observed 100 days after HCT.