Leber's disease is a hereditary condition primarily affecting young men. The mechanism of inheritance is unknown. Increased tortuosity and capillary microangiopathy in the peripapillary capillary bed occur in varying degrees in asymptomatic persons in families with Leber's disease. These vascular abnormalities signify an increased risk of developing the acute form of the disease. Progressive microangiopathy is a threatening sign during the presymptomatic stage. In the acute and atrophic stages of Leber's disease striking neurovascular changes take place in the fundus of the eye involved. The ophthalmoscopic observations and nerve function studies in the asymptomatic, presymptomatic and acute stages suggest that Leber's disease starts as a vascular disease. The neuropathy appears later, around the time that vision begins to fail. Both eyes are involved but at varying intervals. At the end stage the patient has bilaterally finger counting vision and a large centrocecal scotoma caused by severe optic atrophy. Examinations of other persons in families with Leber's disease have shown that subclinical and mild forms of the disease also exist. Neurological signs and symptoms can occasionally occur. Cardiac abnormalities such as pre-excitation syndrome have been reported in Leber's disease. The aetiology and precipitating factors and effective treatment to prevent blindness in Leber's disease remain unsolved questions.