The long-term effects of childhood cancer and its therapy are serious problems that deserve attention. One of the most important late effects is the development of secondary malignancy. We encountered a girl with neuroblastoma who developed acute monoblastic leukemia as a secondary malignancy, 32 months after starting treatment for the primary tumor at the age of 4 years and 10 months. For the primary tumor, she had received cyclophosphamide, ifosphamide, etoposide, epirubicin, cisplatin, and vincristine during a period of 20 months; no radiotherapy was given. Cytogenetic analysis of the leukemic cells showed no specific changes, but a rearrangement of the mixed lineage leukemia gene (chromosome 11q23 translocation) was subsequently found by reverse transcription polymerase chain reaction. The survival time after onset of the secondary malignancy was brief. The leukemogenic hazards of cancer treatment should be weighed against their therapeutic benefits.