The 4-year incidence of blindness and vision loss was examined in a population-based study of diabetes mellitus. In subjects participating in baseline and 4-year follow-up examinations, the rate of blindness was 1.5, 3.2, and 2.7% in younger onset persons, older onset persons taking insulin, and older onset persons not taking insulin, respectively. The rate of blindness increased with increasing age, increasing diabetic retinopathy severity, and lower baseline visual acuity in all three groups. Blindness increased with increasing duration of diabetes in younger onset persons and older onset persons taking insulin. The incidence of vision loss, as measured by a doubling of the visual angle, was associated with older age, more severe retinopathy, and presence of macular edema in the three groups. It was also associated with duration of diabetes, presence of proteinuria, and higher glycosylated hemoglobin in younger onset and older onset persons taking insulin.