INTRODUCTION: Mitochondrial disease is a potential diagnosis in patients with epilepsy beginning in childhood or adolescence with a typical polymorphic presentation and preponderant occipital lobe seizures. Diagnosis may however be delayed in some patients with long-standing disease, particularly when cardinal mitochondrial symptoms are missing; clinical manifestations may be dissociated over time leading to genetic diagnostic tests being prescribed long after disease onset. OBSERVATION: We report the case of a 17 year old woman in whom the diagnosis of lipothymic episodes, migraine, idiopathic photo-sensitive generalized epilepsy, and partial occipital epilepsy complicated by occipital epileptic status were successively proposed because of the initial clinical presentation and the slow disease course. Eleven years after disease onset the diagnosis of progressive myoclonic epilepsy was made due to the occurrence of myoclonic jerks with giant SEPs associated with a cerebellar syndrome, deterioration of psychomotor performances and diffuse slowing of EEG activity with pseudo-periodic bursts of delta waves. Genetic analysis showed an A3243G mutation of mitochondrial DNA, usually correlated with the MELAS phenotype, while the clinical presentation of progressive myoclonic epilepsy was more suggestive of MERRF. CONCLUSION: Although each of the symptoms successively observed in this patient has been reported in MELAS, the slow course of the disease, which is unusual in this mutation, the absence of stroke-like episodes, and the polymorphism of the epilepsy all contributed to delayed final diagnosis.