Localized tumors composed of immature cells of the myelogenous series have been recognized for many years as an uncommon manifestation of granulocytic leukemia. The histologic diagnosis of chloroma (granulocytic sarcoma) may be extremely difficult when the myeloblastic cells are poorly differentiated and the tumor lacks the characteristic green color. The diagnostic difficulty may be further compounded when the granulocytic sarcoma develops before there is peripheral blood or bone marrow evidence of leukemia. Previous criteria for the diagnosis of chloroma have been ambiguous because of the capricious nature of the hydroperoxidase activity and the lack of definitive histochemical criteria. In this case, a combination of Sudan black B and myeloperoxidase histochemical staining and ultrastructural evaluation was applied. The light microscopic histochemical studies suggested the presence of Phi bodies and rods both in the formalin-fixed tumor and in the cells derived from the subsequent pleural effusion; this was confirmed by electron microscopy, which demonstrated the peridicity of the crystalline rod substructure. These observations show that light microscopic histochemical studies can facilitate the diagnosis of granulocytic sarcoma or chloroma in the absence of peripheral blood or bone marrow manifestations of leukemia.