A variable white blood cell count without clinical signs of sepsis or hematological disorders is often observed in newborns. The intention of this study was to investigate the effect of mode of delivery on hematologic measurements, especially white blood cell count in the umbilical vein. 121 term newborns were investigated prospectively. They were subdivided into three groups: spontaneous births (n = 83), vacuum extraction (n = 19) and cesarean sections (n = 19). The mode of delivery influenced the leukocyte count and the precursor cells. The mean leukocyte counts after spontaneous birth (14.6 ± 4.0 leukocytes/nl) and after vacuum extraction (16.6 ± 7.4 leukocytes/nl) were significantly higher than after cesarean section (12.1 ± 4.4 leukocytes/nl; p < 0.05). The precursors of leukocytes were also significantly higher in infants with vacuum extraction (1.06 ± 0.83 cells/nl) than in spontaneously delivered infants (0.54 ± 0.47 cells/nl). An inverse relationship between the number of leukocytes and the arterial cord blood pH was observed (p < 0.05). No significant differences were observed in the effect of mode of delivery on erythrocyte and platelet counts.