Barth Syndrome (BTHS) is an X-linked disorder characterized by severe mitochondrial dysfunction, skeletal and cardiomyopathy and growth retardation. The investigators have recently found severe exercise intolerance in adolescents with BTHS that was mediated by impaired skeletal muscle oxygen extraction and utilization. Previous evidence from other mitochondrial pathologies demonstrated that chronic aerobic exercise training enhanced mitochondrial biogenesis, improved skeletal muscle oxygen extraction/utilization, exercise tolerance and quality of life in these individuals. Chronic aerobic exercise training also improved left ventricular and cardio-autonomic function and decreased the occurrence of arrhythmias in non-Barth heart failure and arrhythmia human and animal models. Currently it is unknown if chronic aerobic exercise training is effective in improving left ventricular function, skeletal muscle mitochondrial biogenesis and oxygen extraction/utilization, exercise tolerance, cardio-autonomic function and quality of life in those with BTHS; a condition containing characteristics consistent with both mitochondrial myopathy and heart failure. Establishing the safety and efficacy of aerobic exercise training in BTHS could lead to clinical recommendations of regular exercise training for the standard of care treatment of individuals with BTHS. It may also provide novel mechanistic information about the adaptability of muscle mitochondria in BTHS. Therefore, the overall objective of the pilot/feasibility/proof-of-concept proposal is to collect preliminary data on the following hypothesis: Supervised aerobic exercise training (3x/wk, 20-45 min, 12 wks) will improve skeletal muscle oxygen extraction/utilization, left ventricular function, peak exercise tolerance, cardio-autonomic function and quality of life, and will be found safe in adolescents and young adults with BTHS. The investigators aim to address these hypotheses through left ventricular function, skeletal muscle oxygen extraction/utilization, and whole body oxygen consumption measurements during a graded exercise test at baseline and following a 3 month supervised aerobic exercise training program in 5 BTHS patients (ages 15-30 yrs). Cardio-autonomic function will be examined using post-exercise heart rate recovery measurements obtained at baseline and after the 12 wk intervention. Supervised exercise training programs will be uniformly designed, but individualized and performed at a hospital based physical therapy or cardiac rehabilitation facility near the participant's home. Left ventricular function will be examined using 2-D, Doppler and Tissue Doppler echocardiography, skeletal muscle oxygen extraction/utilization will be measured using near infrared spectroscopy, whole body oxygen consumption will be measured using indirect calorimetry, cardio-autonomic function will be measured using electrocardiography and quality of life will be measured by the Minnesota Living with Heart Failure Questionnaire (MLWHFQ). The investigators expect to find that exercise training is safe in BTHS, and effectively improves cardiac and skeletal muscle function and quality of life. Preliminary data from this proposal will be used in larger federal or association grant applications examining the cardiovascular, musculo-skeletal and autonomic effects of chronic aerobic exercise training in BTHS.