On clinical examination, an eight-year-old Warmblood mare with history of intermittent fever of unknown origin, showed fever (40.2 Celsius), and painful submandibular lymphadopathy. Hematologic findings revealed pancytopenia, hyperglobulinemia and large, coarse chromatin leukemic cells. Polyclonal hyperglobulinemia was shown on protein electrophoresis. Bone marrow evaluation exposed hypercellular population of >30% blast cells. Painful pitting edema developed on the distal limbs and the mare was euthanized. Submandibular and retropharyngeal hemorrhagic lymphadenopathy, mild edema of limbs and erosive typhlitis in cecum were found on necropsy. Hematopoetic neoplasia in the bone marrow, spleen and mesenteric lymph node, extramedullary hematopoiesis in the submandibular lymph node and lymphangectasia in the skin of legs were found on histopathology. Morphologic evaluation supported the initial classification of an acute leukemia. Cytochemical staining of blood smears were inconclusive. Immunophenotyping with flow cytometry using CD172a fluorescence revealed a myeloid leukemia.