A 6-week-old child presented with hypotonia, myopathy, and a rapidly worsening dilated cardiomyopathy with severe atrial and ventricular arrhythmias and pulmonary hypertension, which proved fatal at age 3 months. Biochemical analysis showed a combined deficiency of the enzymatic activities of complexes I and IV and molecular studies identified a T14709C mutation in the mitochondrial tRNA glutamic acid gene. A review of symptomatology in patients with this mutation shows that it mainly presents in childhood or young adults with mild myopathy and diabetes mellitus. Infants with a high, nearly homoplasmic mutant load can present with more severe symptoms including cardiomyopathy. Families with this mitochondrial DNA mutation should be aware that increased mutant load in a subsequent generation may result in severe and often fatal cardiac symptoms.