OBJECTIVE To describe the relation of retinal arteriolar and venular caliber to the incidence and progression of diabetic retinopathy in people with type 1 diabetes mellitus. METHODS Incidence findings in a population-based study of diabetic retinopathy in Wisconsin. Participants included 996 persons diagnosed as having diabetes mellitus before 30 years of age who took insulin and underwent the baseline examination, 891 in the 4-year follow-up, 765 in the 10-year follow-up, and 634 in the 14-year follow-up. Retinal photographs of 7 standard fields were taken at all examinations. Computer-assisted grading was performed from a digitized image of field 1 to determine the average diameter of retinal arterioles and venules and their ratio. Main outcome measures included incidence and progression of retinopathy, incidence of proliferative retinopathy, and macular edema. RESULTS While adjusting for other factors, larger arteriolar (relative risk [RR] for the fourth vs first quartile range, 2.04; 95% confidence interval [CI], 1.20-3.47; test of trend, P =.008) and venular diameters (RR, 2.33; 95% CI, 1.37-3.95; test of trend, P =.005) were associated with greater 4-year progression of retinopathy. Larger venular diameters (RR, 4.28; 95% CI, 1.50-12.19; test of trend, P =.006) but not arteriolar diameters were associated with greater 4-year incidence of proliferative retinopathy. In multivariable analyses, arteriolar and venular calibers were not associated with the 4-year incidence of retinopathy. While adjusting for other factors, arteriolar and venular calibers were not associated with incidence of macular edema at 4 years. There were few associations of arteriolar or venular caliber with the 10- or 14-year incidence or the progression of retinopathy. CONCLUSIONS Larger arteriolar and venular caliber, independent of retinopathy severity level, is related to the progression of retinopathy, and larger venular caliber is associated with the 4-year incidence of proliferative retinopathy. Caliber of retinal vessels is not associated with incident retinopathy. These data suggest a quantitative measure of retinal vascular caliber provides additional information regarding risk for progression of retinopathy.