Background: Cardiomyopathy is well recognized in mitochondrial diseases in which it has been associated with defects of mitochondrial function, including cytochrome-c oxidase (COX) deficiencies. This study explores the respiratory chain activity, particularly of COX, in patients with cardiomyopathy to determine whether a relationship exists between respiratory enzyme activity and cardiac function. Methods and Results: Myocardial specimens from the left ventricular wall of explanted hearts were obtained from subjects with ischemic (n = 6) or nonischemic dilated (n = 8) cardiomyopathy. Assays for citrate synthase (CS) and complexes II/III and IV activity were performed on cardiac mitochondria and homogenate. Enzyme activities were normalized to CS activity and compared with control activities (n = 10). A significant reduction in COX and/or CS activity was identified in mitochondrial preparations from the transplant group and correlated significantly with ejection fraction (P < .05), although this does not prove a causal relationship. Significantly reduced CS activity in homogenate was identified, suggesting decreased mitochondrial volume in addition to decreased COX activity. Measurements in cardiac homogenates failed to show a significant reduction in COX activity (P > .05) in the transplant group, suggesting that the use of prefrozen tissue homogenates may underestimate existing mitochondrial respiratory defects in cardiac tissue. Conclusions: Mitochondrial function is altered at a number of levels in end- stage cardiomyopathy. Defective COX activity resulting in deficient adenosine triphosphate generation may contribute to impaired ventricular function in heart failure. Agents capable of improving mitochondrial function may find an adjuvant role in the treatment of cardiac failure.