There is a consistent inverse association between birth weight and systolic blood pressure; however, few studies have been able to examine the immediate postnatal period. We have examined whether accelerated postnatal growth predicts adult systolic and diastolic blood pressure. We followed up participants from the Barry Caerphilly Growth Study. Blood pressure data were obtained on 679 of the original 951 subjects (73%) aged approximately 25 years. Both multivariable linear regression and spline models were used to examine the association among weight, length, and growth velocities with systolic blood pressure and diastolic blood pressure. Both statistical approaches showed that birth weight was inversely associated with systolic blood pressure. Only the spline models found that immediate (0 to 5 months) weight gain (beta coefficient: 1.29 mm Hg; 95% CI: 0.36 to 2.23; P=0.007) and weight gain between 1 year and 9 months to 5 years (beta coefficient: 1.44 mm Hg; 95% CI: 0.31 to 2.57; P=0.01) were independently associated with systolic blood pressure, whereas only immediate weight gain (beta coefficient: 0.74 mm Hg; 95% CI: 0.08 to 1.41; P=0.03) was associated with diastolic blood pressure. This is the first study to demonstrate that only immediate postnatal growth predicts diastolic blood pressure in term births, whereas it adds further evidence that both birth weight and postnatal growth are associated with systolic blood pressure in support of both the fetal origins and growth acceleration hypotheses.