A 28-year-old woman presented with a right breast mass and axillary lymphadenopathy. Biopsy of the breast mass revealed myeloid sarcoma (MS) staining positive for CD4, CD13, CD33, and CD68/KP-1. Bone marrow aspiration revealed leukemic cell infiltration (9%). Leukemic cells possessed cytogenetic abnormalities of +8 and t(9;11)(p22;q23) with +22 (lymph node only), and molecular analyses confirmed the MLL-AF9 fusion gene. After induction chemotherapy and 2(nd) consolidation therapy, complete remission was maintained. However, during consolidation radiotherapy for the breast mass, the disease progressed in both the breast and bone marrow. She received re-induction therapy and proceeded to allogeneic stem cell transplantation. However, the disease relapsed in the breast soon after transplantation, and she died from disease progression. Trisomy 8 and the MLL-AF9 fusion gene have been reported in cases with MS in the breast. Trisomy 22 found additionally and exclusively in the extramedullary lesion implies extramedullary progression of MS from the medullary site of origin and may have been associated with the distinctive therapy resistance of these lesions in our case.