Involvement of the pulmonary vasculature by carcinoma of the breast typically occurs in the form of microscopic tumor emboli involving the small arteries, arterioles, or capillaries. Obstruction of a large pulmonary artery by a tumor embolus has not been reported. We describe a patient with a history of breast carcinoma diagnosed 5 years previously who sought treatment for dyspnea and a large mass in the right pulmonary artery suggestive of a pulmonary embolus. After failure of both systemic and intraarterial thrombolytic therapy, a biopsy of the mass was obtained, which revealed adenocarcinoma of the breast. Systemic chemotherapy with doxorubicin and cyclophosphamide was initiated and resulted in the complete resolution of her symptoms.