An 11-month-old, intact female Great Dane presented to the Cornell Emergency Service for a one-week history of non-weight-bearing lameness of her right hindlimb. Radiographs from the primary veterinarian identified aggressive bone lesions of the right femoral metaphyses. She was transferred to the Orthopedics Service for diagnostic workup; bloodwork revealed polycythemia, azotemia, and severe hypercalcemia. A CT scan revealed erosive lesions affecting both femurs and left scapula, a soft tissue mass caudal to the right stifle, and a mediastinal soft tissue mass. Smear evaluation of aspirates from the stifle mass returned with high suspicion of lymphoma. She declined clinically despite supportive care in hospital, and the owners elected euthanasia. The patient was submitted for necropsy, which made an initial gross morphologic diagnosis of metastatic thymic lymphoma. Further histopathological examination and immunohistochemistry led to a revision of this initial diagnosis to one of T lymphoblastic leukemia/lymphoma.