Myeloid sarcoma (MS), also known as chloroma (owing to its green color attributed to the enzyme myeloperoxidase), is a pathologic diagnosis for an extramedullary proliferation of blasts of one or more of the myeloid lineages that disrupt the normal architecture of the tissue in which it is found. It has also been addressed as granulocytic sarcoma, myeloblastoma and extramedullary myeloid cell tumor. Myeloid sarcoma represents the tissue mass form of acute myeloid leukemia (AML), thus, the diagnosis is equivalent to a diagnosis of AML. Myeloid sarcoma may occur de novo, may precede or coincide with AML, or may represent a blastic transformation of a preceding myelodysplastic syndrome or chronic myeloproliferative neoplasm. Myeloid sarcoma may also be the initial manifestation of relapse in a patient with previously diagnosed AML. While diagnosis of MS in an already diagnosed AML patient is easier, the diagnosis of primary MS can be challenging for a pathologist. In one historic study, there is a 75% rate of misdiagnosis. Chloroacetate esterase, myeloperoxidase, and nonspecific esterase should be included in the cyto-chemical stains. Immuno-phenotyping can be performed on paraffin sections or through FACS analysis, using cell suspension obtained from the tumor.