The diagnosis of Leber's hereditary optic neuropathy, essentially based on a family history of a similar illness, becomes difficult in sporadic cases. In this disease, mitochondrial DNA mutations have been recently discovered and detected by digestion with restriction enzyme after amplification by the Polymerase Chain Reaction. We present two patients in whom these molecular biological techniques confirmed the diagnosis of Leber's hereditary optic neuropathy in the absence of a convincing family history. These new molecular biological methods, providing a precious diagnostic test, would certainly play a greater part in genetic counselling and would probably be a prerequisite for the development of new forms of therapy.