We assessed the associations between body mass index (BMI) and blood pressure (BP) with the retinal microvasculature among preschool-aged children. Three hundred and seventy-nine children aged 3-<6 years old had gradable retinal photographs, and quantifiable retinal vascular caliber using validated computer-based methods. Weight, height, BMI, waist circumference and BP were obtained using standardized protocols. Mean (± s.d.) BMI and systolic BP in preschool-aged children were 16.0 (± 1.8) kg m-2 and 93.2 (± 9.9) mm Hg, respectively. After multivariable adjustment, each kg m-2 increase in BMI was associated with a 1.06 μm narrowing of retinal arteriolar caliber (P=0.01) and 1.12 μm widening of retinal venular caliber (P=0.02). Children in the ≥95th percentile (obese) compared with those in the <85th percentile (healthy weight) had narrower retinal arteriolar caliber (˜6.1 μm, Ptrend=0.01) and wider retinal venular caliber (˜6.4 μm, Ptrend=0.01). Each 10 mm Hg increase in systolic BP was associated with a 1.70 μm narrower retinal arteriolar caliber (multivariable-adjusted P=0.02). We show that BMI and systolic BP have inverse linear associations with retinal arteriolar caliber, and BMI has a positive linear association with retinal venular caliber during early childhood. These findings suggests that the influence of BP and BMI on small vessels are continuous and commence early in life.