Purpose: Aim of the study was to evaluate the effects of ethambutol therapy in visual functions. Methods: Prospective evaluation of 88 eyes of 44 patients on ethambutol therapy under Directly Observed Treatment Short-course (category I) for primary tuberculosis was done before start of ethambutol therapy and after 2 months of starting the therapy. Parameters evaluated were visual acuity with Bailey Lovie Log-MAR chart, contrast sensitivity with Pelli-Robson contrast sensitivity chart, color vision with Farnsworth D15 test, visual fields with Octopus automated perimetry, and multifocal electroretinography (ERG) with Roland-RETI scan along with anterior and posterior segments evaluation. Results: No visual functional defect was noted at baseline. On follow-up, color vision, visual field parameters, and anterior and posterior segment findings were not affected in any patients. Mean visual acuity before starting therapy was 0.00±0.08 Log-MAR and after therapy was 0.08±0.18 Log-MAR. Change in visual acuity was statistically significant (p=0.004). Difference between contrast sensitivity before and after therapy was statistically highly significant both monocularly and binocularly (p<0.005 in both cases). P1 amplitudes (in terms of nV/deg 2 and μV) of ERG waves were significantly reduced and their P1 latencies were significantly increased in all the rings after ethambutol therapy (p<0.05). There was no significant change in N1 amplitudes and N1 latencies after therapy in any of the rings. Conclusions: Visual acuity, contrast sensitivity, and multifocal ERG are sensitive tests to detect ethambutol toxicity in subclinical stages and hence very useful tools for monitoring patients under ethambutol therapy for ocular toxicity.