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Purpose: To evaluate the multifocal electroretinography (mfERG) changes in patients on ethambutol therapy., Methods: A cross-sectional observational study of 17 visually asymptomatic patients receiving antituberculosis therapy with ethambutol. Patients underwent complete ophthalmic examination and mfERG recordings. The first-order mfERG N1 and P1 response amplitudes and implicit times of six concentric rings were analysed and compared with 17 normal age-similar controls. Correlation analyses were performed between the patients' mfERG parameters with parameters of ethambutol usage (daily dose of ethambutol per body weight, duration of ethambutol therapy, cumulative dose of ethambutol, and cumulative dose of ethambutol per body weight)., Results: The mean duration of ethambutol therapy was 3.6 months (range: 2-9 months) and the mean daily dose per body weight was 13.2 mg/kg/day. Analysis of response amplitude measures showed no significant difference in the mfERG N1 and P1 response amplitudes between the ethambutol and control groups at all ring eccentricities (P>0.05). For implicit times, there were significant delays in the mfERG P1 implicit times of rings 4-6 in the ethambutol group compared with controls (P=0.012 to P=0.041). Correlation analyses showed no significant correlation between the mfERG and ethambutol parameters (P>0.05)., Conclusions: The mfERG findings suggested that visually asymptomatic patients receiving ethambutol therapy might have localized mild electrophysiological changes involving the peripheral macula. These changes might be related to localized alteration of metabolism or physiological changes associated with ethambutol therapy.