Acute respiratory failure in the perioperative period represents a frequent challenge to the anesthesiologist. The differential diagnosis is extensive and includes alterations on the pulmonary parenchyma, pulmonary vessels, airway, and cardiac system. Occasionally, two or more pathophysiological process superimpose. We present a patient who suffered from a left pulmonary embolism that was appropriately diagnosed and treated. However, the hypoxemia persisted and a second pathology was suspected. After careful evaluation and differential diagnosis, we drained a right pleural effusion, which had been present preoperatively, with resolution of the hypoxemia. There is controversy in the literature as to the role of drainage of pleural effusions on improving oxygenation. We present this case as an example of successful management of perioperative respiratory failure by thoracentesis in the presence of a second concurrent pathologic process.