The most important problem in the approach to young patients with acute pleurisy is distinguishing those with idiopathic or viral pleurisy from patients with pulmonary embolism. Three clinical features are helpful in making this distinction: (1) pleural effusion(s) present on chest roentgenography, (2) history of predisposing factors for or past history of veno-occlusive disease, and (3) physical signs indicative of phlebitis. Lung scanning should be performed in patients with these findings. If results of scanning are highly characteristic of pulmonary embolism (segmental or lobar defect with ventilation/ perfusion mismatch) in such a patient, anticoagulation may be considered immediately. Patients in whom scanning reveals indeterminate characteristics or abnormalities not characteristic of pulmonary embolism should undergo pulmonary angiography if other clinical features suggest that the probability of pulmonary embolism remains at least moderately high.