A 59-year-old man with pulmonary tuberculosis took antituberculous drugs including ethambutol (EB) for 6 months without side effects. His visual acuity reduced 3 months after completion of the therapy, deteriorated rapidly in the next 2 months, and was finally almost lost. After we found three members of his family with Leber's hereditary optic neuropathy (LHON), it was diagnosed in the present case by confirming the mitochondrial DNA mutation specific to it. LHON takes matrilineal inheritance form and causes peculiar degeneration of retinal ganglion cells and acute or subacute reduced visual acuity. Although it usually develops in youths, the present middle-aged man had not developed LHON until the treatment including EB. We speculate that the administration of EB triggered LHON in the present case. EB has been frequently used for mycobacterial therapy, and therefore we should inquire about a patient's family history concerning eye disease before its administration.