Azacitidine is a first-in-class demethylating agent, and it is widely used globally as a first-line treatment for higher-risk myelodysplastic syndrome (MDS). Here, we report the case of a patient with MDS who suffered from a rare adverse event, an acute lung injury (ALI), which was suspected to have been caused by azacitidine and was successfully treated with corticosteroids. As it is a rare, but critical, adverse event, clinicians should consider ALI as one of the differential diagnoses in cases where 1) pneumonia and fever of unknown etiology arise in MDS patients treated with azacitidine, 2) antimicrobial agents are not effective, and 3) microbiological tests produce negative results.