Acute lymphoblastic leukemia (ALL), in general, can be diagnosed by detecting blasts in peripheral blood or bone marrow. Some of the cases of ALL do not show typical leukemic features, and only manifest as refractory anemia, thrombocytopenia, myelofibrosis and lymphocytic infiltration into bone marrow. Several months after presentation, they may reveal typical leukemic features and are diagnosed as ALL. This kind of leukemia is called ALL with aleukemic prodrome. Although the incidence of ALL with aleukemic prodrome is 1.5~2.2% of childhood ALL cases, it is rarely reported in Korea. We experienced a 6 month-old female infant who presented with refactory anemia and thrombocytopenia, and two serial of bone marrow examination revealed only myelofibrosis. She subsequently developed ALL 3 months later. We report this case with a brief review of related literatures.