Secondary lymphoid malignancy, particularly acute lymphoblastic leukemia (ALL), is rare. We report one case of ALL and another case of mediastinal lymphoblastic lymphoma developed after treatment for neuroblastoma. The secondary ALL characterized by short latency period and an 11q23 translocation apparently was induced by etoposide treatment. The pathogenesis of the secondary lymphoma is less certain and may be related to previous treatment with cyclophosphamide and radiotherapy, host susceptibility, or chance occurrence. One child died of progressive lymphoma and the other remains in remission 1 year following allogeneic bone marrow transplantation. Additional studies are needed to determine the risk, pathogenesis, and optimal treatment for secondary lymphoid malignancy.