We present the case of a child with acute lymphoid teukemia (ALL) who was morphologically classified as FAB L1 (PAS and peroxidase were negative). Remission was achieved with an ALL-type protocol (GBTLI). Five months after the discontinuation of therapy, the patient presented mixed leukemia (CD10, CD19,CD13 and CD33 were positive) with (9;11) (p21;q23) translocation. Unfortunately, as cytogenetic and immunophenotype studies were not performed at diagnosis, two possibilities could be considered for the relapse; secondarly mixed leukemia with clonal chromosome changes, or mixed leukemia from the beginning.