Pulmonary tumor embolism is a rare but well-documented cause of respiratory failure in patients with-cancer. This entity is probably clinically underrecognized and may represent an important cause of morbidity and mortality. A typical pattern of multiple peripheral subsegmental defects on perfusion lung scanning has been described. We present a case of a unilateral near absence of perfusion in a 53-year-old woman who on autopsy was found to have diffuse tumor microembolism. As new chemotherapeutic agents are developed for the treatment of malignancies, an early diagnosis may become more important. A high-probability perfusion scan should not dissuade the clinician from pursuing further evaluation. Cytologic findings of pulmonary venous blood may be diagnostic.