Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes is a progressive, multisystem mitochondrial disease affecting children and young adults. Patients acquire disability through stroke-like episodes and have an increased mortality. Eighty per cent of cases have the mitochondrial mutation m.3243A>G which is linked to respiratory transport chain dysfunction and oxidative stress in energy demanding organs, particularly muscle and brain. It typically presents with seizures, headaches and acute neurological deficits mimicking stroke. It is an important differential in patients presenting with stroke, seizures, or suspected central nervous system infection or vasculitis. Investigations should exclude other aetiologies and include neuroimaging and cerebrospinal fluid analysis. Mutation analysis can be performed on urine samples. There is no high quality evidence to support the use of any of the agents reported in small studies. This article summarises the core clinical, biochemical, radiological and genetic features and discusses the evidence for a number of potential therapies.