If the finding of the spiral CT scan is negative for pulmonary emboli, and for pleural, parenchymal, and mediastinal lesions, and if the patient is improving, I believe observation is the best course of action. Patients with malignancy rarely improve spontaneously. If the patient is not improving or is getting worse, attempts should be made to obtain a diagnosis. Since the panel of tumor markers, as outlined by Porcel and associates, will identify only 54% of the patients with tumor, a negative finding on a panel of tumor markers does not rule out malignancy. Therefore, an invasive procedure is indicated to determine the etiology of the pleural effusion.