A 74-year-old Japanese man with myelodysplastic syndrome (MDS) received chemotherapy with azacitidine. From the second day after starting the administration, he complained of fever, cough and shortness of breath. Chest roentgenography and computed tomography showed consolidations and ground-glass opacities. His symptoms grew from worse to life-threatening. We diagnosed him with azacitidine-induced pneumonitis and began administering corticosteroids. Thereafter, his symptoms and radiographic abnormalities improved. Azacitidine is a hypomethylating agent that improves the survival of MDS patients. Although this drug is commonly well tolerated and rarely causes severe lung injury, it is important to consider the potentially serious adverse effects of azacitidine-induced pneumonitis.