Retinal vascular caliber has been linked to diabetic retinopathy (DR). Newer imaging technologies allow analysis of retinal vascular caliber beyond the standard areas surrounding the optic disc. We investigated the vascular caliber in extended zones in prediction of DR in adolescents with type 1 diabetes. Prospective, longitudinal study. Adolescents (n = 904) who attended the diabetes complications assessment service at the Children's Hospital at Westmead. Retinal caliber was assessed from baseline retinal photographs with a semiautomated computer software Singapore I Vessel Assessment: standard zone retinal vessel calibers were summarized as central retinal arteriolar equivalent (CRAE) and central retinal venular equivalent (CRVE), and extended-zone vessels (>2 disc diameters from the optic disc margin) were summarized as mean width of arterioles (MWa) and mean width of venules (MWv). Retinal vessel calibers at baseline (upper 3 quartiles vs. lowest quartiles [Q2-4 vs. Q1]) and moderate DR (stage 3 or more) were analyzed using multivariate generalized estimating equations, with results expressed as odds ratios (OR) and 95% confidence intervals (CIs). Among the 904 participants, baseline mean age ± standard deviation was 14.0 ± 1.5 years, hemoglobin A1C (HbA1C) level was 8.5 ± 1.3%, and median diabetes duration 4.6 years. After a median 3-year follow-up, 15% of adolescents demonstrated moderate DR. Wider extended-zone retinal arteriolar caliber (MWa; OR, 3.6 [95% CI, 2.06-6.1], comparing Q2-4 vs. Q1) and venular caliber (MWv; OR, 4.2 [95% CI, 2.2-7.5]) predicted moderate DR after adjusting for HbA1C and blood pressure. Standard zone CRAE and CRVE were not associated with moderate DR. Extended-zone retinal vessel caliber predicts moderate DR in adolescents with type 1 diabetes.