We describe an 8 year old boy who had received chemotherapy for an acute promyelocytic leukemia and developed a secondary leukemia 27 months after the diagnosis of this first malignancy. Blasts cells were positive for cytoplasmic markers CD22, CD3 and myeloperoxidase. Cell surface T and myeloid-associated markers were also detected. Cytogenetic study disclosed monosomy 7. The patient achieved complete remission, but relapsed 15 months later with identical immunophenotypic and cytogenetic findings. Three-lineage commitment is proved by the expression of specific criteria for myeloid, and lymphoid T and B typing. A multipotent immature progenitor must be the target of leukemogenic agents. The prognosis is obviously ominous.