The authors retrospectively reviewed complete blood counts in the medical records of all babies admitted to a normal newborn nursery from January through December 1989. The subjects consisted of 326 babies who were delivered vaginally (VgD) and 138 who were delivered by Cesarean section (CS). All blood samples were drawn by warmed or unwarmed heel sticks or by venipuncture. The subject's age at the time of blood drawing was similar in both VgD and CS groups (13.4 vs. 13.9 hours, P = 0.51). The number of total leukocytes, neutrophils, band forms, and platelets was significantly higher in VgD newborns than in CS newborns. The mean and standard error of the mean for each of these blood counts (each per microliter) were 23.9 x 10(9) +/- 0.33 versus 21.1 x 10(9) +/- 0.6, 14.6 x 10(9) +/- 0.26 versus 12.8 x 10(9) +/- 0.39, 1.18 x 10(9) +/- 0.08 versus 0.82 x 10(9) +/- 0.08, and 304 x 10(9) +/- 4.1 versus 286 x 10(9) +/- 0.6, respectively (P values for the first three comparisons were all less than 0.005). However, there was no difference between the two groups with regard to hemoglobin, hematocrit, and absolute number of lymphocytes, eosinophils, basophils, and monocytes (P > 0.5). It was speculated that the higher leukocyte, neutrophil, and band counts in VgD babies are the consequences of physical stress and periodic hypoxia, which are more frequent and prolonged with VgD compared with CS delivery. The authors suggest that the mode of delivery should be considered when interpreting blood counts in neonates.