Background—Low birth weight predisposes to later coronary disease. To further elucidate the mechanisms behind this association and their timing, vascular endothelial function—a key factor in early pathophysiology of atherosclerosis—was studied in 54 infants born either before the third trimester or at term.Methods and Results—All subjects were studied at 3 months of postnatal age. A laser-Doppler technique was used to measure skin perfusion before and after transdermal iontophoresis of acetylcholine (ACh; an endothelium-dependent vasodilator). In infants born at term (n=19; birth weight range: 2230 to 4205 g), maximum perfusion after ACh was 109±8 perfusion units (PU, mean±SEM) in normal–birth weight controls compared with 56±13 PU among those who had been small for gestational age at birth (P<0.01). In infants born preterm (n=35; birth weight range, 722 to 1868 g), ACh induced similar perfusion responses among subjects appropriate for gestational age (113±16 PU) and in those small for gestational age at birth (109±19 PU).Conclusions—Impairment in human endothelial function associated with low birth weight occurs or emerges late in pregnancy. Very preterm birth attenuates this association. Different gene–environment interactions in the third trimester may contribute to this finding.