The octanoylation of Ser3 is essential for the biological function of ghrelin. We examined the concentrations of the n-octanoylated active-form ghrelin in cord and neonatal blood using an RIA system that specifically recognized n-octanoylated ghrelin, as well as a system that measured the total ghrelin concentration. Plasma levels of active ghrelin in cord blood ranged from 7.7 to 38.4 pmol/l and correlated excellently with those of total ghrelin (r = 0.81, p<0.0001). The active ghrelin/total ghrelin (A/T) ratio ranged from 0.038 to 0.12 (median 0.072). The active ghrelin concentrations negatively correlated with birth body weight (r = –0.34, p = 0.01) and IGF-1 concentrations (r = –0.40, p = 0.003), but did not correlate with growth hormone (GH) concentrations. A considerable level of active ghrelin was detected in premature newborns. Venous cord blood samples showed a significantly higher active ghrelin concentration (p = 0.03) and A/T ratio (p = 0.01) than those in the artery. In neonatal blood, active ghrelin concentrations ranged from 4.6 to 22.6 pmol/l and the A/T ratio ranged from 0.02 to 0.081. These results demonstrate the existence of active-form ghrelin in fetal and neonatal circulation and may suggest the energy supply-dependent regulation of ghrelin expression/secretion in utero.