Ethambutol is frequently used in the treatment of tuberculosis, and, although optic neuropathies have been reported with the use of ethambutol, this adverse side effect has been considered to be rare and generally reversible with discontinuation of the medication. However, we recently saw two patients with renal tuberculosis treated with ethambutol in whom visual loss from toxic optic neuropathies was severe and irreversible despite careful ophthalmological monitoring and prompt discontinuation of the agent at the first sign of impaired visual function. While ethambutol treatment is most commonly instituted for pulmonary tuberculosis, it is interesting to note that both of these patients had renal tuberculosis. Since ethambutol is actively excreted via the renal system, compromise of renal function such as due to renal tuberculosis may lead to serum concentration elevations of ethambutol sufficient to produce optic neuropathy.