Abstract The clinical and pathologic features of seventeen patients with the carcinoid syndrome are reviewed. Nine had carcinoid heart disease. The only clinical feature which distinguished those patients with carcinoid heart disease from those without was the presence of a precordial systolic murmur suggestive of tricuspid regurgitation or pulmonic stenosis. Urinary excretion of 5-hydroxyindoleacetic acid was similar in the two groups. The chest roentgenogram and the electrocardiogram was of no help in determining presence or absence of carcinoid heart disease. Cardiac catheterization in patients with carcinoid heart disease has shown that tricuspid regurgitation and pulmonic stenosis are the most frequent valvular alterations. Cardiac output appears to be increased in some patients who do not have carcinoid cardiac involvement. The cardiac lesions associated with the carcinoid syndrome were pathognomonic and bore little resemblance to lesions found in other cardiac diseases. Cardiac involvement in the carcinoid syndrome consisted of focal or diffuse collections of a peculiar type of fibrous tissue, which was free of elastic fibers and which was deposited on the endocardium of the valvular cusps, on the endocardium of the cardiac chambers and on the intima of the great veins, coronary sinus and occasionally great arteries. The valvular cusps per se remained normal as did the mural endocardium, and each was clearly separated from the fibrosing process by the elastic membrane which covered its surface. Left-sided cardiac lesions were commoner in the carcinoid syndrome than previously thought and were found in the absence of a right to left shunt or a pulmonary carcinoid tumor. The pathogenesis of the carcinoid cardiac lesions (carcinoid fibrous plaques) remains unknown. The morphologic appearance of the endocardial lesion suggests that it might be the result of the deposition of a material, possibly collagen, from the blood, but the identity of the substance which either directly or indirectly produced the endocardial fibrosis is unknown. The inability to produce these cardiac lesions in animals with serotonin and the present observation that the urinary excretion of 5-hydroxyindoleacetic acid was similar in the patient with carcinoid heart disease irrespective of the presence or absence of cardiac involvement suggest that mechanisms other than that involving serotonin should be considered.