We describe the cases of two patients who presented with granulocytic sarcoma with mediastinal involvement 15 and 21 months before development of acute myeloid leukemia. In both cases several bone marrow aspirates and trephine biopsy specimens, obtained at presentation and subsequently, revealed no evidence of leukemic infiltration. One case was originally misdiagnosed as large-cell non-Hodgkin's lymphoma, which resulted in inappropriate therapy. In both cases immunohistochemical staining revealed that tumor cells were positive for leucocyte common antigen but not for conventional B- or T-lymphoid-cell markers. Retrospective analysis revealed that tumor cells in both cases were positive for myeloid markers. Histopathologists should be aware that granulocytic sarcoma may occur in unusual extramedullary sites without evidence of bone marrow involvement. If inappropriate treatment is to be avoided, a diagnosis of granulocytic sarcoma should be considered when hemopoietic tumor cells do not stain with conventional antibodies against B- and T-lymphoid cells. Both histochemical and immunohistochemical staining should be performed in such cases to determine whether the cells are of myeloid lineage. A diagnosis of granulocytic sarcoma is not ruled out when bone marrow biopsy specimens show no evidence of leukemic infiltration.