Myeloid sarcoma, also known as granulocytic sarcoma or chloroma is an unusual accumulation of malignant myeloid precursor cells in an extramedullary site, which disrupts the normal architecture of the involved tissue. It is known to occur more commonly in patients with acute myelogenous leukemia and less commonly in those with myelodysplastic syndrome and myeloproliferative neoplasm, such as chronic myelogenous leukemia. The most common sites of involvement include bone, skin and lymph nodes. However, rare cases have been reported in the gastrointestinal tract, genitourinary tract, or breast. Most commonly, a neoplastic extramedullary proliferation of myeloid precursors in a patient would have systemic involvement of a myeloid neoplasm, including in the bone marrow and peripheral blood. Infrequently, extramedullary disease may be the only site of involvement. It may also occur as a localized antecedent to more generalized disease or as a site of recurrence. Herein, we present the first case in the English literature of a patient presenting with an isolated site of myeloid sarcoma arising in the form of a colonic polyp which, after subsequent bone marrow biopsy, was found to be a harbinger of chronic myelogenous leukemia. Myeloid sarcoma, also known as granulocytic sarcoma or chloroma is an abnormal accumulation of malignant myeloid precursor cells in an extramedullary site, which disrupts the normal architecture of the involved tissue. It is known to occur more commonly in patients with acute myelogenous leukemia and less commonly in those with myelodysplastic syndrome and myeloproliferative neoplasm, such as chronic myelogenous leukemia. The most common sites of involvement include bone, skin and lymph nodes. However, rare cases have been reported in the gastrointestinal tract, genitourinary tract, or breast. Most commonly, a neoplastic extramedullary proliferation of myeloid precursors in a patient would have systemic involvement of a myeloid neoplasm, including in the bone marrow and peripheral blood. Infrequently, extramedullary disease may be the only site of involvement. It may also occur as a localized antecedent to more generalized disease or as a site of recurrence. Herein, we present the first case in the English literature of a patient presenting with an isolated site of myeloid sarcoma arising in the form of a colonic polyp which, after subsequent bone marrow biopsy, was found to be a harbinger of chronic myelogenous leukemia.