Lineage switch from acute lymphoblastic leukemia (ALL) to acute myeloid leukemia (AML) is very rare. We report a case of a 9 yr-old ALL patient relapsed as acute myelomonocytic leukemia. At the initial diagnosis, the blast cell morphology and immunophenotype were consistent with the diagnosis of typical ALL (L1 subtype according to FAB classification). The BCR-ABL fusion gene was not found by reverse transcription-PCR. Complete remission (CR) was achieved after induction and consolidation chemotherapy (Children's Cancer Study Group 1891 protocol, CCG1891). Nine months, which is a very short time compared with other cases in the literatures, after the diagnosis of ALL, she relapsed with completely different blasts (typical AML, M4 according to FAB classification) in morphology, cytochemistry, and immunophenotyping. The karyotype has changed from 56,XY,+X,+Y,+Y,+4,+8,+10, +14,+17,-20,+21,+21,+21[6]/57,idem,+Y[19] to 46,XY,t(8;16)(p11.2;p13.1)[19]/46,XY[1], showing unrelated chromosomal abnormality to the karyotype at the initial diagnosis. Moreover, both findings were quite specific for each common cell ALL and acute myelomonocytic leukemia. These findings support that this case is completely different leukemic clones occurred at each leukemic expression. The treatment with AML 2000 protocol chemotherapy failed, and he underwent the chemotherapy with the combination of high dose cytarabine and mitoxantrone and has been in CR state for 21 months, until now.