Low birthweight is associated with increased risk of hypertension, but underlying mechanisms are obscure. We hypothesized structural microvascular alterations may be one such mechanism. We examined the association of birthweight and retinal arteriolar caliber in 3800 persons aged 51 to 72 years participating in a population-based study in 4 US communities (the Atherosclerosis Risk in Communities study). Participants reported full-term birth and their birthweight and had retinal photography. Retinal arteriolar and venular calibers were measured from digitized retinal photographs using a validated computer-assisted method. Lower birthweight was associated with narrower retinal arteriolar caliber, with each kg lower birthweight associated with 2.4 mum (95% confidence intervals, 1.3 to 3.5, P<0.001) narrower retinal arteriolar caliber, after controlling for age, gender, race, education, smoking, alcohol consumption, adult body mass index, and height. Additional adjustment for blood pressure averaged over the past 6 years and fasting glucose did not alter these findings, with each kg lower birthweight associated with a 1.7 mum (95% confidence intervals, 0.7 to 2.8, P<0.001) narrower retinal arteriolar caliber. This association was also present in persons without hypertension or diabetes. Lower birthweight was not associated with retinal venular caliber. These findings provide evidence that structural alterations in the arteriolar microcirculation may be a potential mechanism linking restricted fetal growth with subsequent risk of hypertension.