Maternal smoking in pregnancy is associated with respiratory diseases in the offspring, possibly due to prenatal influences on the developing immune system. We investigated whether maternal smoking in pregnancy was associated with cord blood leukocyte numbers, including precursor dendritic cells, adjusting for concomitant factors. In a prospective healthy birth cohort study, total leukocyte counts were reduced in neonates of smoking mothers [10.7 (8.4-13.0), n=14] compared with nonexposed infants [14.7 (13.7-15.7), n=74, p=0.002] [geometric mean cells x 10(3)/microL (95% confidence interval)]. All leukocyte subsets were decreased, most prominently segmented neutrophils [4.3 (2.8-5.7) versus 6.2 (5.5-6.8), p=0.021], lymphocytes [3.8 (2.9-4.8) versus 5.0 (4.5-5.6), p=0.036], and myeloid precursor dendritic cells [12.7 cells/microL (9.1-17.8) versus 18.3 (15.8-21.2), p=0.055]. These differences persisted after adjustment for possible confounders. Predictors of myeloid precursor dendritic cell numbers in multivariable models were maternal smoking (-5.1 cells/microL, p=0.042), age (-0.5 cells/microL/y, p=0.035), and, marginally, asthma (+8.1 cells/microL, p=0.075). The decrease of all leukocytes in neonates of smoking mothers could be clinically significant and suggests a decreased cell production, increased peripheral recruitment, or retention in bone marrow. Given the importance of dendritic cells in early immune responses, their decrease might reflect an impact of maternal smoking on the developing fetal immune system.