Contrast sensitivity was measured by means of Arden grating plates in 100 patients on medication for pulmonary tuberculosis. The scores were abnormal in 38.2% of the patients whose therapy included ethambutol for three months, and 36.7% of the patients on similar treatment for six months. In comparison with age matched groups of patients on a regimen where streptomycin replaced ethambutol a statistically significant number of the patients on ethambutol had abnormal scores. This study suggests that Arden contrast sensitivity plates would be effective in detecting subclinical toxic optic neuropathy due to ethambutol and therefore could be used for routine monitoring of ocular function of patients on ethambutol. Loss of contrast sensitivity may explain why some patients on ethambutol with normal visual acuity and colour perception may still complain of visual disturbance.