Mitochondrial disorders associated with defects in the respiratory chain can be attributable to mutations in the mitochondrial genome (mitochondrial DNA) or the nuclear genome (nuclear DNA). Because the brain is highly dependent on oxidative metabolism, encephalopathy is a common presentation, and epilepsy is a clinical hallmark of many of these conditions. Although most mutations in mitochondrial DNA do not present in infancy, a few mutations in the adenosine triphosphatase gene cause maternally inherited Leigh disease and infantile epilepsy. Early-onset epilepsy is more commonly associated with defects of nuclear genes encoding subunits of respiratory chain complexes or proteins needed for the correct assembly and functioning of the complexes. These defects generally cause autosomal recessive Leigh disease. In this review, the frequency and types of epilepsy (particularly early-onset seizures) are compared according to a genetic classification of the mitochondrial disorders.