Granulocytic sarcoma is an uncommon tumor composed of granulocytic precursor cells. Because it occurs in a variety of clinical settings and because the tumor cells are primitive it is frequently unrecognized during life. This presentation details the authors' experience with 61 biopsy-proven granulocytic sarcomas. The patient age range was from 2 to 81 years (mean 48 years). In eight patients the tumors were multiple. Most common sites of involvement were bone, periosteum, soft tissue, lymph node and skin. Twenty-two tumors occurred in 15 patients with no known disease, 26 occurred in 24 patients with a known myeloproliferative disorder, and 13 occurred in 11 patients with proven acute myeloid leukemia. Thirteen of the 15 patients with no known disease developed acute leukemia in from one to 49 months after the biopsy of their tumors (mean 10 months). Most tumors occurring in patients with a known myeloproliferative disorder were associated with blast crisis. The authors' cases displayed a morphologic range from well-differentiated to those tumors that displayed virtually no evidence of differentiation by conventional microscopy. It was therefore not surprising that most tumors were originally diagnosed as lymphoma. Chloro-acetate esterase (CAE) stains were performed on 56 tumors and 47 were studied with antilysozyme immunoperoxidase technique. Fifty-six of the 57 specimens studied by either technique were positive. Antilysozyme immunoperoxidase stains were particularly useful in confirming the diagnosis.