We report a case of severe aplastic anemia (SAA) treated with granulocyte colony-stimulating factor (G-CSF), cyclosporin A and danazole, in which myelodysplastic syndrome (MDS) with monosomy 7 developed eight months later. A 24-year-old woman was diagnosed as having SAA and was initially treated with G-CSF, cyclosporin A and danazole. At initial presentation, bone marrow aspirate revealed marked hypocellularity with a normal karyotype (46, XX [20]) and no MDS features. Eight months after initial treatment, leukocytosis and reticulocytosis were observed and bone marrow aspirate showed hypercellular marrow with morphological and cytogenetic features (45, XX, -7 [16]/46, XX [4]) characteristic of MDS (refractory anemia). A total of 75 mg (1.25 mg/kg) of G-CSF had been administered during the preceding eight months. Among seven previous reports published in Japan since 1992, in which MDS/acute myelogenous leukemia (AML) developed from SAA treated with G-CSF, six showed monosomy 7. Careful observation for leukemic transformation is therefore indicated in patients with SAA who are treated with G-CSF.