The endocardial lesions in carcinoid heart disease are characterized by the presence of smooth muscle cells, myofibroblasts and fibroblasts embedded in a stroma that is rich in collagen, microfibrils and basement membrane material, but lacks elastic fibers. BIBLIOGRAPHIC REFERENCES: Ferrans, V.J., and Roberts, W.C.: The carcinoid endocardial plaque. An ultrastructural study. Hum Pathol 7: 387-409, 1976.