Mitochondrial disease can manifest in many different ways whether in children or adults. Examples of mitochondrial diseases include:
* Mitochondrial myopathy
* Maternally inherited diabetes mellitus and deafness (MIDD)
**  While diabetes mellitus and deafness can be found together for other reasons, at an early age this combination can be due to mitochondrial disease, as may occur in Kearns–Sayre syndrome and Pearson syndrome 
* Leber's hereditary optic neuropathy (LHON)
**LHON is an eye disorder characterized by progressive loss of central vision due to degeneration of the optic nerves and retina (apparently affecting between  1 in 30,000 and 1 in 50,000 people); visual loss typically begins in young adulthood
* Leigh syndrome, subacute necrotizing encephalomyelopathy
** after normal development the disease usually begins late in the first year of life, although onset may occur in adulthood
** a rapid decline in function occurs and is marked by seizures, altered states of consciousness, dementia, ventilatory failure
* Neuropathy, ataxia, retinitis pigmentosa, and ptosis (NARP)
** progressive symptoms as described in the acronym
** dementia
* Myoneurogenic gastrointestinal encephalopathy (MNGIE)
** gastrointestinal pseudo-obstruction
** neuropathy
* MERRF syndrome
** progressive myoclonic epilepsy
** "Ragged Red Fibers" are clumps of diseased mitochondria that accumulate in the subsarcolemmal region of the muscle fiber and appear when muscle is stained with modified Gömöri trichrome stain
** short stature
** hearing loss
** lactic acidosis
** exercise intolerance
* MELAS syndrome, mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes
* Mitochondrial DNA depletion syndrome
Conditions such as Friedreichs ataxia can affect the mitochondria but are not associated with mitochondrial proteins.'