Following spontaneous delivery (n = 12) a pronounced increase in the maternal total white blood cell count was found (to [X ± SD]: 20 ± 6 × 109 total leukocytes/1), which would be considered highly ‘pathological’ in nonpregnant women. The alteration was predominantly due to an increase in polymorph-nuclear cells and band forms. Simultaneously, a drastic decrease in the percentage and the absolute number of lymphocytes was noticed in venous blood (from 32 to 5%, or [X ± SD]: from 2.7 ± 0.7 to 1.1 ± 0.3 × 109 total lymphocytes/1). In this decrease nearly all lymphocyte subtypes were involved, although to differing extents. Following elective caesarean delivery (n = 6), no leukocytosis was found, however the percentage and absolute number of lymphocytes was also decreased, although not as pronounced as after spontaneous delivery. Again not all the lymphocyte subpopulations were affected to the same degree, and the effect was especially obvious for the suppressor T cells and B cells. Following emergency caesarean delivery (n = 5), no obvious effect on the absolute number of lymphocytes or on the pattern of lymphocyte sub-populations was observed. However, the total number of white blood cells was clearly increased, as after spontaneous deliveries. The possible significance of these findings, also for therapeutic consequences during the perinatal period, is discussed.