Dyspnea in a patient with cancer may have several causes, including infection, thromboembolism, metastases, and therapeutically induced cardiopulmonary disease. Pulmonary tumor embolism is an uncommon cause. Occlusion of the pulmonary microvasculature by tumor cells and associated thrombi can produce a subacute and progressive clinical picture that resembles thromboembolic disease. Unfortunately, microscopic tumor embolism is recognized rarely before death because of difficulty in establishing the diagnosis. We provide a review of the literature about the evaluation and diagnosis of this rare clinical entity.