Retinal arteriolar narrowing is a known response of hypertension and independently predicts cardiovascular mortality in adults. Whether elevated blood pressure leads to retinal arteriolar narrowing in young children is unknown. We examined the relationship of retinal vascular caliber and blood pressure levels in 2 population-based cohorts among children aged 6 to 8 years in Sydney, Australia (1572 children) and Singapore (380 children). Participants had digital retinal photographs and measurement of retinal arteriolar (or small artery) and venular (or small vein) caliber. Children with higher quartiles of blood pressure had significantly narrower retinal arterioles than those with lower blood pressure (retinal arteriolar caliber 162.8, 161.0, 157.8, and 157.1 microm (P for trend<0.001), comparing increasing quartiles of systolic blood pressure in Sydney, and 164.9.5, 164.0, 159.1, and 159.4 microm (P for trend=0.0024 in Singapore). After controlling for age, sex, race, body mass index, refraction, and birth parameters, each 10-mm Hg increase in systolic blood pressure was associated with narrowing of the retinal arterioles by 2.08 microm (95% confidence interval: 1.38 to 2.79; P<0.0001) in Sydney children and 1.43 microm (95% confidence interval: 0.27 to 2.59; P=0.016) in Singapore children. These associations were consistent across age, sex, body mass index, and birth parameters. Retinal venules were not affected by blood pressure. We conclude that higher childhood blood pressure is associated with retinal arteriolar narrowing. Our data provide evidence that the effects of elevated blood pressure may manifest early in life.