Fifty patients with Leber's hereditary optic neuropathy (LHON) with an associated pathogenic mutation of mitochondrial DNA (mtDNA) at base pair (bp) 11778 (35 cases), 14484 (eight cases) or 3460 (seven cases) were matched with 50 controls. The frequency of additional neurological features in LHON and the role of a number of past medical and environmental factors in the development of the disease were investigated using a case-control study. Additional neurological features were reported by 15 patients. Four patients had a multiple sclerosis-like illness; one had focal dystonia. Ten patients had tremor, which occurred at significantly higher frequency in patients than in controls. Alcohol and tobacco consumption were similar in patients with the 11778 mutation and matched controls, but were significantly increased in patients with the 3460 and 14484 mutations. No other associated past medical or environmental factors were identified.