A 73-year-old woman was diagnosed with myelodysplastic syndrome(MDS). After 11 courses of treatment with azacitidine( AZA), her hemoglobin level and platelet count improved significantly, and she became transfusion independent. Therefore, treatment was discontinued and follow-ups were maintained. Three months later, her platelet count reduced again; we therefore treated her again with AZA. However, MDS transformed to acute myeloid leukemia in the 14th course, and she died 19 months after the initial diagnosis. AZA is an important drug for treating MDS, but premature withdrawal of treatment might cause rapid disease progression. In case treatment is discontinued, the patient needs to be carefully observed.