The fine structure alterations in the atrium and atrial appendage, mitral valve and papillary muscle are described in 11 matched patients with chronic rheumatic heart disease. The muscle changes consisted of loss of myofilaments and accumulation of lipid and osmiophilic dense bodies. The connective tissue stroma of the atrium and the mitral valve showed extensive deposition of collagen and elastic fibers. There were numerous foci of collagen degeneration, characterized by fraying of the collagen fibers and accumulation of homogeneous granular material at these sites. Although the muscle changes were more striking, the connective tissue alterations appear important in the evolution of the chronic disease. The extent of collagen degeneration appeared to parallel the degree of collagen formation. The muscle fiber degeneration and connective tissue alterations did not correlate with the clinical findings. At the resolution of the electron microscope, the continuing process in the rheumatic heart appears to be primarily collagen formation and degradation rather than primary degeneration of the muscle fibers. It is the balance of these processes which determine the clinical state of the patient. Acute muscle damage along with evidence of inflammation do not seem to be associated with progressive, chronic rheumatic heart disease.