Association Between Retinal Vascular Calibre and Blindness in Young Patients With Type 1 Diabetes Purpose To examine the association between retinal vascular calibre and incident blindness caused by diabetic retinopathy in young patients with type 1 diabetes. Methods A case-control study of 6 patients (12 eyes) who later went blind and 18 age- and sex-matched controls (36 eyes). They were all identified in a population-based cohort study of 339 patients with type 1 diabetes. Patients and controls all participated in a clinical baseline examination in 1995 and were subsequently followed for 15 years. Incident blindness was defined for patients who registered between 1995 and 2010 in the Danish Association of the Blind, which is a voluntary organization open for patients with a visual acuity at or below 6/60 (0.1) in the best eye. Each blind patient was matched with 3 controls regarding age and sex. At baseline, median age of both groups was 21 years (range: 14-23 years for the blind and 15-23 years for the controls (p=0.77)) and median duration of diabetes was 13 years for those who later went blind (range: 9-15 years) and 12 years for the controls (range: 9-16 years) (p=0.25). Retinopathy was graded according to the EURODIAB-Hammersmith protocol. Baseline level of retinopathy for those who became blind was: no retinopathy: 3 eyes, mild non-proliferative retinopathy (NPDR): 3 eyes, moderate NPDR: 2 eyes, severe NPDR in 2 eyes and proliferative retinopathy in 2 eyes. Among controls baseline retinopathy ranged between no retinopathy (20 eyes, 55.6%), mild NPDR (15 eyes, 41.6%) and moderate NPDR (1 eye, 2.8%). From baseline retinal photos, central retinal artery and vein equivalent (CRAE and CRVE) was calculated in the validated semi-automated computer program IVAN using the Big6 method. Two eyes (one in each group) were ungradable for IVAN due to quality of photographs. Wilcoxon rank-sum test was used to analyze the association between retinal vascular calibre and blindness. Results The median age and duration of diabetes at which the patients became blind was 27 years and 20 years, respectively. As compared to controls, patients who later became blind had significantly greater median CRVE, 263.00 m vs. 228.81 m (p=0.008). Median CRAE (164.5 m vs. 156.7 m, p=0.17) did not differ between patients who went blind and controls. Conclusions We found that type 1 diabetic patients who later went blind had a larger baseline CRVE than controls, despite the low number of patients. This indicates that CRVE may be an early marker of severe retinopathy.