The aim of the study was to examine the relationship of retinal vascular calibre with glucose intolerance, diabetes and retinopathy in a population-based cohort.The Australian Diabetes, Obesity and Lifestyle study recruited adults aged > or =25 years old from across Australia. Participants were classified using an oral glucose tolerance test as having normal glucose tolerance (NGT), impaired glucose tolerance (IGT), impaired fasting glucose (IFG), known diabetes or newly diagnosed diabetes. Digital retinal photographs were taken of all participants with diabetes, IGT and IFG, and a sample of those with NGT, and graded for the presence of retinopathy. Retinal vascular calibre was measured from photographs by a computer-assisted method.Of the 1,998 participants with gradable retinal images, 16% had known diabetes, 17% newly diagnosed diabetes, 42% IGT, 6% IFG and 19% NGT. After multivariable adjustment, retinal arteriolar calibre was significantly larger in people with known diabetes (178.9 microm) compared with participants with NGT (174.6 microm, p = 0.02), IGT/IFG (175.5 microm, p = 0.02) or newly diagnosed diabetes (175.6 microm, p = 0.047). One SD increase in mean arteriolar calibre was associated with higher odds of diabetes compared with NGT (odds ratio [OR] = 1.28, 95%CI = 1.06-1.55). After multivariable adjustment, each SD increase in venular calibre was associated with higher odds of having retinopathy in persons with IGT/IFG (OR = 1.78, 95%CI = 1.36-2.34) or in persons with diabetes (OR = 1.68, 95%CI = 1.23-2.29).Diabetes is associated with larger retinal arteriolar calibre and retinopathy with larger retinal venular calibre. The contrasting associations may reflect different underlying pathophysiological processes in the natural history of diabetes.