The objective of this paper is to test the hypothesis that the 2-h hematocrit (HCT) is lower in infants born by cesarean section (CS) than in infants born vaginally and that the postnatal rise of HCT is lower in infants born by CS than in infants delivered vaginally. We prospectively studied 31 infants delivered by elective CS, 21 infants delivered by CS because of arrest of descent and 30 vaginally delivered (VD) infants. All pregnancies were term, uncomplicated in nonsmoking mothers. Apgar scores were > 7 at 1 and 5 min. In all infants umbilical cord was clamped early. Umbilical vein and 2-h peripheral venous micro-HCT were measured by centrifugation. Cord-blood HCT, 2-h HCT, and rise in HCT were similar in the two CS groups and significantly lower than in the VD group. In multiple regression, gestational age, Apgar score, or the presence or not of labor did not influence cord-blood HCT, 2-h HCT, or the rise in HCT. Infants born by CS have lower HCT than infants born vaginally. Prediction of 2-h HCT from cord-blood HCT must take into account the mode of delivery.