Neurological investigations, HLA-typing and viral antibody studies were performed in patients with Leber's optic neuropathy (LON), in individuals at risk to develop the disease, in obligatory (female) carriers of the disease, and compared with controls. The only relevant findings were an excess of minor neurological abnormalities in patients with LON and in some individuals from the at risk group. Occasionally, an association of LON and a multiple sclerosis-like picture was observed.