In general, mitochondrial disease is a progressive disorder. A substantial number of children with mitochondrial disease die before the age of 10 years. The disease can affect patients severely, but there are also mild variations. The rate of decline is unpredictable and very long stable periods may occur.

Most patients eventually develop involvement of several organ systems. Since mitochondrial disease involves a very heterogeneous group of patients, no general statements about the prognosis of these patients can be made. Prognosis is dependent mainly on which and how severely organs are affected, and on the progression rate, which is very individual.

The degree in which a patient with mitochondrial disease is ill depends on the production of energy. An infection or another external factor may have a negative impact on the energy levels and as a result may deteriorate the situation of a mitochondrial patient for a longer period. An innocent attack of the flu may cost so much energy that it takes a long time before the patient is back at his/her earlier level of health. Sometimes this level is never recaptured again.

There is no obvious relationship between the age of onset of the disease and the level of severity of the disease. Unfortunately, children with a progressive and serious mitochondrial disease often die at an early age. It happens, but only rarely so, that the clinical situation of a patient improves over time.