Abstract: Breast cancer has a predilection for spreading to the bone, brain, liver and lung, however metastasis to bone marrow resulting in bone marrow failure is considered rare. Here, we discuss a case of breast cancer presenting with bone marrow involvement and diffuse bone lytic lesions. The patient was an81 year old female presenting with back pain in the lumbar region for four months, progressively worsening despite physical therapy and oral analgesics. She was referred for magnetic resonance image which revealed diffuse bone lytic lesions. Follow up computed tomography of chest, abdomen and pelvis confirmed bone lesions but was negative for any primary malignancy. Peripheral blood studies showed white blood cell count of 4.5x 10³/µL, hemoglobin of 6.6g/dL, hematocrit of 21% and platelet count of 120.000/µL. She also had renal dysfunction with creatinine of 1.41mg/dL and calcium of 9.8mg/dL. Due to concern for a plasma cell neoplasia, the patient was referred to our oncology clinic. Physical examination was unremarkable and peripheral blood studies revealed IgG 1411mg/dL, IgA 292mg/dL, IgM 122mg/dL with undetectable serum and urine M spikes. She underwent a bone marrow biopsy which was negative for multiple myeloma but showed a neoplastic component in the marrow (approximately 5%) positive for Pan-Keratin, GATA3, ER and Cyclin D1, consistent with mammary carcinoma. During further questioning, she reported a normal screening mammogram one year prior to the onset of symptoms. Positron emission tomography (PET)was remarkable for extensive bony metastatic disease and a heterogeneous hyper-metabolic adrenal mass concerning for metastasis. She was started on endocrine therapy with a daily aromatase inhibitor and monthly Denosumab for bone metastasis. At her six month follow up, PET-scan showed stable disease. Currently, she remains on the same hormonal regimen with monthly follow up at the oncology clinic.