AbstractAlthough it is well recognized that granulocytic sarcoma can cause localized lymphadenopathy, widespread nodal involvement by acute myelocytic leukemia (AML), clinically mimicking non‐Hodgkin's lymphoma, has only been previously described twice. We report the clinicopathological, immunological, and cytochemical features of two patients who had widespread, prominent lymphadenopathy secondary to AML as well as concurrent marrow leukemia (M1 and M2). For one patient the lymphadenopathy was the predominant abnormality prompting him to seek medical attention, while the second patient had symptoms of infection following a 9‐month history of myelodysplasia. The disease in both patients was aggressive; one patient survived only 1 week and the other survived only 5 weeks after diagnosis. In both cases the granulocytic sarcoma was confirmed by cytochemistry studies (naphthol ASD‐chloroacetate esterase on tissue sections and myeloperoxidase on imprint smears), and electron microscopy, including morphology (both cases) or ultrastructural localization of myeloperoxidase (case 2). Non‐specific esterase activity was not detected in either patient's blasts, although serum lysozyme was elevated in both cases. Immunological studies revealed reactivity of both patients' cells with panleukocyte, MY4, MY7, OKM‐1, and Leu‐M1 monoclonal antibodies and with alpha‐1‐antitrypsin and muramidase antibodies. The cells of one of these patients also reacted wtih anti‐S‐100 protein. Although the cytochemical studies indicated that both cases exhibited only myeloid differentiation, the immunological markers suggested that the tumor cells possessed some features of monocytes, perhaps explaining their propensity for widespread tumor formation. Morphological, immunological, cytochemical, and ultrastructural methods of diagnosing granulocytic sa