Dyspnea and hypoxemia in a patient with cancer may have several causes, including infections, thromboembolism, metastases, and pulmonary injuries by drugs. We report a 47 year-old female with breast cancer and a 70 year-old male with urinary bladder cancer who were admitted for dyspnea, hypoxemia and pulmonary hypertension. Chest X rays and CT scans were normal. The ventilation-perfusion scintigram was highly suspicions of thromboembolism in the female. The male died 16 hours after admission and the post mortem examination revealed the presence of tumor cells in the pulmonary vasculature (Rev Med Chile 2009; 137: 1613-16).