Children with Down syndrome (DS) are at a significantly increased risk of acute leukemia. We present a rare case of de novo acute lymphoblastic leukemia associated with DS (DS-ALL) after remission of myeloid leukemia associated with Down syndrome (ML-DS). At the age of 18 months, a girl with DS had anemia and thrombocytopenia. Her bone marrow aspirate showed 17% blasts expressing CD41, CD7, CD13, CD33, and GP-A, and having the GATA1 mutation. She was diagnosed as having ML-DS and was treated for AML, and she then showed long-term remission. At the age of 7 years, she had petechiae and thrombocytopenia again. Her bone marrow aspirate showed 96% blasts expressing CD10, CD19, CD20, and HLA-DR. However, these leukemic cells did not have the GATA1 mutation. She was diagnosed as having de novo DS-ALL and received treatment for ALL. Since the age of 14 years, she has been in complete remission of both ML-DS and DS-ALL.