We describe a new glare test that uses low contrast letters and a variable surround glare source. The test is easy to administer clinically and all test targets, luminance levels, glare geometry, and viewing conditions are well controlled. We found the most useful index of disability glare to be the difference in visual acuity scores for the low contrast chart in the no-glare and high-glare conditions and refer to this as the disability glare index (DGI). We tested a group of normal subjects who were divided into younger (age 15 to 41 years) and older (age 50 to 82 years) groups. The mean DGI value for the older group (10.2 +/- 4.8) was significantly higher than that for the younger group (2.3 +/- 1.9). The DGI is a better discriminator between the two groups than either high or low contrast visual acuity. We attribute the significantly higher DGI values in the older group to increased intraocular light scatter. We find DGI is poorly correlated with high contrast visual acuity (r = 0.33). Our findings suggest that this test of disability glare is sensitive to relatively modest changes in the ocular media. It is a potentially useful tool in detection and assessment of subtle media disturbances and in monitoring changes in the ocular media over a period of time.