A 39 year-old patient suffered bilateral painless visual loss while consuming abusively alcohol and cigarettes. Family history was remarkable for a similar episode in the patient's brother 21 years ago, while drinking and smoking heavily. A diagnosis of toxic optic neuropathy was made in another institution, but due to the positive family history, mitochondrial DNA was studied for a point mutations associated with Leber's hereditary optic neuropathy. A homoplasmic mutation at nucleotide 11778 was found. The patient's brother could not be studied. This case emphasizes the probable deleterious effects of epigenetic factors such as alcohol and cigarette smoke in patients harbouring mitochondrial DNA mutations associated with Leber's hereditary optic neuropathy and the need for mitochondrial DNA studies in atypical cases of optic neuropathy.