Glare disability plays an important part in the investigation and assessment of cataract. We investigated the effect of monocular glare disability on binocular contrast sensitivity. The magnitude of binocular summation was measured at different degrees of monocular glare disability. In the absence of glare, maximum binocular summation was shown. With increasing glare disability, the degree of binocular summation decreased steadily until binocular inhibition was produced. Binocular inhibition is defined as a reduction in binocular sensitivity to reach a level below the monocular. The clinical implications of binocular inhibition with monocular glare disability are discussed.