A 46-year-old woman presented with a large left breast mass. FDG PET/CT demonstrated a large left breast mass (SUV = 28). Preoperative biopsy showed malignant sarcoma. Modified radical mastectomy was performed. Immunohistochemical analysis using h-caldesmon and smooth muscle actin staining established the diagnosis of primary breast leiomyosarcoma. Despite intensive chemotherapy, 3-month follow-up PET/CT revealed a large lung metastasis suggesting aggressiveness and poor response to chemotherapy. Primary leiomyosarcoma of the breast is extremely rare and difficult to diagnose before excisional surgery because of the need for immunohistochemical staining to confirm the diagnosis.