Lymphoma cells present within the subendothelium and subendothelial capillaries of a large pulmonary vein. The patient was a 72-year-old woman who presented with multiple strokes of unclear etiology. Ultimately a brain biopsy revealed the diagnosis. She subsequently became increasingly tachypneic and developed multi-lobar pneumonia. The presence of lymphoma cells within the alveolar capillaries, interrupting gas exchange, can explain the patient's respiratory symptoms.