PURPOSE To evaluate contrast sensitivity function in patients with impaired oral glucose tolerance test (OGTT) compared with normal subjects. METHODS Sixteen patients with impaired OGTT and 11 normal control subjects were tested for contrast sensitivity function at four spatial frequencies. Glucose intolerance was established by the 1985 World Health Organization criteria. RESULTS The two groups were similar in terms of age, visual acuity, refractive correction, and lens opacities. A statistically significant loss of contrast sensitivity was associated with impaired OGTT (p < 0.001) in every spatial frequency tested. CONCLUSIONS Functional visual loss in patients with impaired glucose tolerance using the 1985 World Health Organization criteria indicates that at least part of those patients should be classified as diabetic according to the 1997 American Diabetic Association criteria. It seems that the 1997 American Diabetic Association criteria are more efficient at detecting patients with abnormal visual function.