Visual acuity was measured in a population-based study of diabetic retinopathy in southern Wisconsin. Persons diagnosed prior to 30 years of age and taking insulin (younger onset, n = 996) and those diagnosed at 30 years of age or older (older onset, n = 1370) were examined. Best corrected visual acuity was determined using the Early Treatment of Diabetic Retinopathy Study protocol. In the younger onset group, 1.4% had moderate visual impairment (best corrected visual acuity in the better eye of 20/80 to 20/160) and 3.6% were legally blind (visual acuity in the better eye of 20/200 or worse). Visual impairment in this group was associated with older age at examination, longer duration of diabetes, presence of proliferative retinopathy, and presence of senile cataracts. In the older onset group, 3.0% had moderate visual impairment and 1.6% were legally blind. Visual impairment in this group was associated with older age at examination, longer duration of diabetes, presence of senile cataract, presence of macular edema, and proliferative diabetic retinopathy. When assigning causes of impaired vision, diabetic retinopathy was responsible in part for 86% of eyes with visual acuity of 20/200 or worse in younger onset persons and for 33% in older onset persons.