We report a rare case of microscopic pulmonary tumor embolism from breast cancer in a young woman with no prior medical history. The patient presented with progressive dyspnea and questionable abnormalities of the left breast and axilla on physical examination. Results of an axillary lymph node biopsy and subsequent radiologic studies raised the possibility of either microscopic pulmonary tumor emboli or lymphangitic spread of cancer as a cause of the dyspnea. The patient died due to advanced breast cancer confirmed at autopsy despite chemotherapy.