Visual acuity, colour vision, and contrast sensitivity of diabetic and non-diabetic subjects were measured before, 20 minutes after, and 24 hours after exposure to slit-lamp illumination either alone or during argon laser photocoagulation. In some instances a significant deterioration of these visual functions was noted when the tests were repeated 20 minutes after light exposure, but by 24 hours after light exposure visual function of all groups had returned to pre-exposure levels. The visual acuity was the most systematically affected by the procedure; there were wide interindividual differences in the changes observed in the other tests.