Tissue sections from hearts of dogs suffering from cardiomyopathies were studied histologically and ultrastructurally. Two types of mitochondrial changes were defined and quantitated. Mitochondrial hypertrophy occurred in cardiomyocytes of middle-aged and older dogs. Significant numbers of degraded cardiomyocytes related to hydropic degeneration of hypertrophied mitochondria associated with autolysis of contractile elements and loss of cellular physiochemical functions were seen in hearts of old dogs with cardiomyopathy. Dogs with cardiac disease that were treated to enhance dysfunctional energy production early in the course of disease had cellular changes similar to those seen in old dogs but they survived heart disease and ultimately died of other causes.