A 45-year-old man with acute myelogenous leukemia (WHO classification, AML with multilineage dysplasia) received allogeneic bone marrow transplantation from an HLA-identical brother in first remission. He became febrile on day 7, and pulmonary failure and multi-organ failure developed subsequently, requiring mechanical ventilation. Chest X-ray and CT scan demonstrated diffuse interstitial shadows, suggesting the development of idiopathic pneumonia syndrome. Administration of methylprednisolone and tacrolimus was effective, but respiratory failure exacerbated along with a decrease in the dose of steroids. Lung biopsy revealed organizing pneumonia with CMV pneumonia. Methylprednisolone and mycophenolate mofetil were instituted, which led to an improvement of lung injury. Intensive immunosuppressive therapy with mechanical ventilation should be considered for the treatment of idiopathic pneumonia syndrome after allogeneic bone marrow transplantation.