Three patients with clinical features of pulmonary embolism were evaluated by chest roentgenograms, perfusion lung scanning, and pulmonary arteriography. All of them had hilar and/or mediastinal masses which were obvious in one patient, subtle in another, and not discernible in the third. The three patients had abnormalities on lung scans, including unilateral absence of perfusion in two and lobar absence of perfusion in one. The extent of the perfusion defects correlated poorly with the roentgenographically visible masses. Pulmonary arteriograms were abnormal but none showed evidence of pulmonary embolism. Lung scanning is often unreliable in establishing the diagnosis of pulmonary embolism in patients with hilar or mediastinal masses. Pulmonary arteriography may demonstrate the mass if it is vascular, or may show extrinsic compression upon the pulmonary vasculature. Lung scanning may be of value in localizing a mass not visible on chest roentgenogram in a patient suspected of harboring a bronchogenic carcinoma.