Background. Leber’s hereditary neuropathy of the optic nerve (LHON) is manifested by bilateral affection of the eyes with acute or subacute loss of vision. The disease is caused by point mutations in the mitochondrial DNA (mtDNA) and is one of the most frequent mitochondrial diseases in the population. In patients with LHON 18 different point mutations in the mtDNA were described which correlate partly with the rate of progression of the disease and the severity and prognosis of the final affection of vision. Methods and Results. The submitted paper deals with the results of molecular genetic examinations in three families with clinical manifestations of LHON. In three patients in the first family a homoplasmic mutation of mtDNA G3460A was found. In the second family in a young man with severely impaired vision a heteroplasmic mutation G3460A was found associated with a higher ratio of mutated mtDNA molecules than in his mother who is clinically healthy. In the third family the presence of homoplasmic mutation of mtDNA in position G11778A was detected. Conclusions. The diagnosis of LHON and genetic counselling in affected families should be based on close collaboration of ophthalmological and genetic departments with specialized laboratories engaged in molecular biological diagnosis of mitochondrial diseases.

        Key words: blindness, LHON, mtDNA, mutation G3460A, mutation G11778A, heteroplasmia.