Cardiac dysfunction due to systemic sarcoidosis is most of ten due to severe restrictive pulmonary diseases. Although the diagnosis is frequently missed during life, direct granulomatous infiltration of the myocardium may occur with systemic sarcoidosis and, when present in the heart, is a major cause of death. To explore the possible use of radioactive 201thallium, a new agent for myocardial imaging, for improved clinical recognition of sarcoid heart disease, myocardial perfusion imaging with 201Tl was performed in six normal volunteers and in five patients with documented systemic sarcoidosis and clinically apparent cardiac dysfunction. Two of the patients with sarcoidosis had severe restrictive pulmonary disease. Their myocardial perfusion scans revealed relatively uniform uptake of 201Tl by the left ventricle, similar to that found in the normal volunteers; however, the right ventricular uptake of 201Tl and right ventricular thickness in these two patients with sarcoidosis was greater than normal. The other three patients with sarcoidosis had unexplained congestive heart failure, mitral regurgitation, or arrhythmias. Myocardial perfusion imaging in these patients revealed normal right ventricular uptake of radioactivity but segmental defects in the left ventricle compatible with an infiltrative disease of the myocardium. Segmental myocardial infiltration by sarcoid was confirmed by autopsy in one of these patients and at operation in another. Thus, myocardial imaging with 201Tl may provide a noninvasive technique for the improved clinical recognition of primary myocardial sarcoid and for distinguishing it from cardiac dysfunction secondary to pulmonary disease.