In 3 young male patients, aged 10, 19 and 21 years respectively, sequential, severe, painless bilateral visual loss occurred. Ophthalmological examination revealed no other abnormalities and this delayed the diagnosis Leber's hereditary optic neuropathy (LHON). LHON is a mitochondrial genetic disease characterised by bilateral acute or subacute painless loss of central vision. LHON causes blindness, predominantly in young adult males but less frequently in women and children as well. Occasionally, LHON is associated with other neurological and cardiac changes. The first patient recovered his vision within 2 years, but the other 2 remained blind. All 3 patients had a m.11778G > A mutation in the mitochondrial DNA (mtDNA). Over 95% of LHON cases are primarily the result of one of three mitochondrial DNA point mutations. In addition, analysis of patients grouped according to mtDNA mutation has demonstrated differences in both the clinical features of visual failure and in recurrence risks for relatives that are associated with each of the pathogenic mtDNA mutations. Depending on the type of mutation, recovery of vision occurs in 4-58% of the patients. Whilst pathogenic mtDNA mutations are required for the development of LHON, other factors must be responsible for the variable penetrance and male predominance. Familiarity with the clinical spectrum of LHON is necessary for early diagnosis. There is no proven treatment.