The relations between size at birth and blood pressure were examined in a population-based longitudinal study of pregnancy and childbirth in the English county of Avon (the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC)) in 1994-1996. A total of 1,860 singleton children aged 3 years (response rate = 74%) were studied. Both height and body mass index were strongly related to blood pressure. After adjustment for current height and body mass index, birth weight showed a graded inverse relation with both systolic (−1.91 mmHg/kg, 95% CI −2.61 to −1.21 mmHg/kg, p < 0.0001) and diastolic blood pressure (−1.42 mmHg/kg, 95% CI −1.96 to −0.88 mmHg/kg, p < 0.0001) which was similar in boys and girls. Although birth length, head circumference, and ponderal index at birth were also inversely related to blood pressure, these relations disappeared after adjustment for birth weight. The strength of the birth weight-blood pressure relation was not strongly influenced by maternal height or by weight gain in the first year after birth, but was particularly strong in children who were shortest at 3 years of age. While the association between birth weight and blood pressure is consistent with reports from many earlier studies, the absence of independent relations between other measures of size at birth (particularly length:head circumference and ponderal index) and blood pressure does not suggest that undernutrition at a critical period of fetal growth plays an important role. Moreover, accelerated postnatal growth does not seem to underlie the birth weight-blood pressure association.