An association between low birth weight and subsequent elevated blood pressure has been demonstrated in a large number of studies, but the number of subjects born small for gestational age in these studies has been negligible. The inverse relationship between birth weight and blood pressure in children has been evaluated previously with an ambulatory blood pressure device, but only in children with normal birth weights. In this prospective case-control study from birth to the age of 12, we evaluated the ambulatory blood pressures in 50 children born at term but small for gestational age and in 50 full-term children born appropriate for gestational age. Children born small for gestational age had similar mean+/-SD systolic (117.5+/-8.5 mm Hg versus 115.3+/-7.4 mm Hg, P=0.221), and diastolic (69.2+/-5.3 mm Hg versus 67.3+/-4.4 mm Hg, P=0.075) 24-hour ambulatory blood pressure compared with the values of the children born appropriate for gestational age. However, 24-hour systolic blood pressure in the small-for-gestational-age children was higher (3.90 mm Hg; 95% confidence interval, 0.65 to 7.15) after adjusting for current body mass index. The difference in current body mass index was the only determinant for the difference in systolic blood pressure between the groups. Birth weight had no direct association with the blood pressure values. Impaired fetal growth may have a relationship with higher later blood pressure, but in 12-year-old children, blood pressure differences between small for gestational age and appropriate for gestational age children are much more dependent on current body size.