This study will evaluate the safety and efficacy of EPI-743 in participants with severe mitochondrial respiratory chain diseases who are considered to be within 90 days of end-of-life care.

Inclusion criteria:

1. Participants with genetic diagnosis: Genetically confirmed diagnosis of Inherited mitochondrial respiratory chain disease
2. Participants with clinical diagnosis: Diagnosis of inherited mitochondrial disease absent genetic confirmation; Specifically, participants must meet the diagnostic criteria of "definite" or "probable" mitochondrial disease as defined by Bernier et al., 2002
3. Deemed by principal investigator to be within 90 days of end-of-life hospice/terminal care
4. Male or female age > one year
5. Hematocrit within normal range for age group
6. Agreement to use contraception if within reproductive years
7. Participant or participant's guardian able to consent and comply with protocol requirements
8. Presence of caregiver to ensure study compliance
9. Abstention from use of all pill-form dietary supplements and non-prescribed medications (except as allowed by the investigator)
10. Abstention from foods or beverages or bars fortified with Coenzyme Q10, vitamin E, super-fortified "functional" foods or beverages
11. Abstention from use of idebenone
12. Clinically staged with a Mitochondrial Disease Scale such as the Newcastle Score

Exclusion criteria:

1. Allergy to EPI-743, vitamin E or sesame oil
2. Clinical history of bleeding or abnormal prothrombin time (PT)/partial thromboplastin time (PTT) (excluding anticoagulation Rx)
3. Hepatic insufficiency with liver function tests (LFTs) greater than two times normal
4. Renal insufficiency requiring dialysis
5. Fat malabsorption syndromes precluding drug absorption
6. Any other concurrent inborn errors of metabolism
7. Severe end-organ hypo-perfusion syndrome secondary to cardiac failure resulting in lactic acidosis
8. Pregnancy