THE usefulness of angiocardiography in evaluating the nature and extent of intra-thoracic lesions is well established. It is frequently used in the preoperative study of primary cardiac malformations, abnormalities of the aorta and great vessels, and in determining the location and operability of pulmonary and mediastinal lesions. In some instances angiocardiography may be the most accurate means of determining whether or not a perplexing mediastinal mass seen on a conventional chest x-ray film is a primary mediastinal lesion or a cardiovascular abnormality. In this communication, a group of difficult diagnostic problems selected from a series of 164 patients with primary tumors and cysts of the mediastinum are presented. Each of these patients was thought, on the basis of the initial examination and chest x-ray films, to have a cardiovascular abnormality. Only after obtaining angiocardiograms was the correct diagnosis of a primary mediastinal lesion made and appropriate therapy instituted. Material A