ObjectiveTo investigate relationships between birth characteristics and blood pressure at age 20 years and to assess whether effects of birth weight on blood pressure are amplified from childhood to adulthood.DesignA longitudinal study of 584 men and women from Adelaide, Australia, examined previously at 8 years and followed up at age 20 years.ResultsBirth weight was negatively associated with systolic pressure at age 20 years in men (regression coefficient 2.6 mmHg per kg; 95% confidence interval 0.7, 4.4) and women (regression coefficient 4.6 mmHg per kg; 95% confidence interval 2.9, 6.4), after adjustment for current weight. There was an interaction with current size (P= 0.05 for men andP= 0.09 for women), such that effects were enhanced among individuals with relatively high weight or weight for height. Shortness at birth, thinness at birth, and low birth weight relative to placental weight were also associated with elevated systolic pressure at age 20 years. Effects of birth weight on blood pressure were stronger at age 20 than at age 8 years (P< 0.01 for men andP= 0.03 for women). This was not due simply to increased variability of blood pressure in adulthood. There were greater rises in blood pressure with age among individuals of relatively low birth weight.ConclusionsThese findings are further evidence that poor fetal growth is associated with elevated blood pressure in later life. The results support the hypothesis that the relationship is amplified with increasing age.