Inherited disorders of energy metabolism are increasingly being recognized as important causes of cardiomyopathy in children. We previously reported that heteroplasmic point substitutions in the mitochondrial DNA D-loop were found in 15 of 75 children at risk for mitochondrial disease (vs 0/95 controls). Four of these cases presented with severe cardiomyopathy in congestive failure in addition to other anomalies and are presented here. In each case, myocardial dysfunction greatly improved following supportive therapy aimed at reversing both congestive failure and catabolism. D-loop point heteroplasmy may be a marker for severe, reversible, infantile multisystem disease that can present with cardiomyopathy.