Although mitochondrial (mt) DNA mutation at nucleotide position 11778 accounts for most cases of Leber's hereditary optic neuropathy (LHON), the phenotypic expression may vary greatly even in different members of the same family. The possible influence of exogenous toxicity on phenotypic expression is still debated in LHON. Here we describe 4 siblings carrying the 11778 mtDNA mutation with a different phenotype. The index case developed an atypical optic neuropathy at the age of 60 years after a long history of occupational exposure to polycyclic aromatic hydrocarbons (PAHs). This report underlines a number of unanswered questions about phenotypic variability of LHON including the possible influence of PAH toxicity.