Mitochondrial diseases occur due to inadequate energy production. In addition, nitric oxide (NO) deficiency occurs in mitochondrial diseases. The endothelial layer of blood vessels functions in maintaining blood vessels patency through producing NO which relaxes vascular smooth muscles and therefore maintains the patency of blood vessels and adequate blood perfusion. In mitochondrial diseases, endothelial cells fail to perform their normal function in maintaining the patency of blood vessels (endothelial dysfunction) because of the inability to produce enough NO. Endothelial dysfunction results in decreased blood perfusion in different organs which can contribute to the complications seen in mitochondrial diseases. The amino acids arginine and citrulline act as NO precursors and can increase NO production. In this study we hypothesize that due to the inability to produce enough NO patients with mitochondrial diseases have endothelial dysfunction that will improve after arginine or citrulline supplementation. The investigators will assess endothelial function using peripheral arterial tonometry before and after arginine or citrulline supplementation. The investigators expect that arginine and citrulline supplementation will improve endothelium function. This will support the therapeutic use of arginine and citrulline in mitochondrial diseases.

The investigators will study 10 children with mitochondrial diseases and 10 healthy control children. The investigators will study control children to be able compare the results of children with mitochondrial diseases to control healthy children. Therefore, the investigators will study control children only once to compare these results to the results of children with mitochondrial diseases at baseline. To assess the effect of arginine and citrulline on children with mitochondrial diseases, the investigators need to assess children with mitochondrial diseases four times. First time as baseline. Then children with mitochondrial diseases will be randomized to receive either oral arginine or citrulline at a dose of 500 mg/kg/day divided in 3 doses daily for 2 weeks after which a second assessment will be performed. Oral arginine or citrulline will be discontinued and after a two-week washout period another baseline assessment will be done. After that, the child will be started on oral citrulline (if the participant received arginine the first time) or arginine (if the participant received citrulline the first time) at a dose of 500 mg/kg/day divided in 3 doses daily for 2 weeks after which a fourth assessment will be done.

Inclusion Criteria:

1. Diagnosis of mitochondrial disease with multi-organ disease involving neurological and/or muscular systems.
2. The diagnosis must be confirmed either molecularly (identifying mutations known to be associated with mitochondrial diseases) or by respiratory chain assay abnormalities.

Exclusion Criteria:

1. Have acute illness or physical disability interfering with ability to undergo the study procedures.
2. Known to have other factors that are known to result in endothelial dysfunction including hypertension, hyperlipidemia, and diabetes.