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A 6-year-old girl with a granulocytic sarcoma (GS) of the left maxillary sinus that followed a uniquely indolent clinical course (3.5 years) and was associated with highly destructive skeletal disease is described. The tumor cells demonstrated an unusual hematogenous "homing" preference for bone and soft tissue sites. Tumor cell characterization with immunologic markers, electron microscopy, cytochemistry, and cytogenetic studies revealed that the tumor cells expressed OKM1 and MMA (Leu-M1), but not HLA-DR, B-, or T-cell markers. The cells were nonspecific esterase- and myeloperoxidase-positive, had ultrastructural features of promyelocytes, and were clonal. The laboratory characterization of the tumor cells in this clinically unusual case of GS illustrates the utility of monoclonal antibodies, applied in conjunction with cytochemistry and ultrastructural analysis, in establishing the specific diagnosis, cell lineage, and maturational stage of this tumor.