Low birth weight is associated with an increased risk of adult hypertension. To elucidate whether this association reflects altered vascular physiology already at birth, we studied acetylcholine-induced vasodilation. Forty newborn infants and their mothers were studied 3 d after delivery. Vasodilation in skin was induced by local application of acetylcholine and local heating to 44 degrees C. Perfusion changes were measured with the laser Doppler technique. In response to acetylcholine, the mean skin perfusion increased by 240% in low birth weight infants compared with 650% in normal birth weight controls (p < 0.001). In contrast, mothers of low birth weight infants showed a mean increase in perfusion of 1100% after acetylcholine administration compared with 680% in mothers of control infants (p < 0.05). The perfusion increase at 44 degrees C local skin temperature did not differ between the two groups of infants or between their mothers. Blood pressure was normal in all subjects. We conclude that low birth weight infants show signs of endothelial dysfunction at birth. Such findings may help us understand the link between low birth weight and adult hypertension.