The authors present the clinical profile of a 6-year-old girl with an unusual immunophenotype of acute lymphoblastic leukemia (ALL). At the initial presentation, massive hepatosplenomegaly developed. The leukemic cells were myeloperoxidase-negative and morphologically lymphoblastic. These cells were positive for B-precursor-cell (CD10, CD19) antigens and natural killer cells (CD56, CD57). Rearrangements of both immunoglobulin heavy chain alleles and monoallelic rearrangement of T-cell receptors (TCRs)-beta and -delta genes, but not that of TCR-gamma gene, were detected, suggesting that these cells being of B-precursor origin. The patient received chemotherapy for extremely high-risk ALL with a good response. To the authors' knowledge, this is the first pediatric case describing coexpression of CD56 and CD57 on B-lineage ALL.