Immunologic effects of pre- and postnatal polychlorinated biphenyl (PCB)/dioxin exposure in Dutch infants from birth to 18 mo of age are explored. The total study group consisted of 207 healthy mother-infant pairs, of which 105 infants were breast-fed and 102 children were bottle-fed. Prenatal PCB exposure was estimated by the PCB sum (PCB congeners 118, 138, 153, and 180) in maternal blood and the total toxic equivalent (TEQ) level in human milk (17 dioxin and 8 dioxin-like PCB congeners). Postnatal PCB/dioxin exposure was calculated as a product of the total TEQ level in human milk multiplied by the weeks of breast-feeding. The number of periods with rhinitis, bronchitis, tonsillitis, and otitis during the first 18 mo of life was used as an estimate of the health status of the infants. Humoral immunity was measured at 18 mo of age by detecting antibody levels to mumps, measles, and rubella. White blood cell counts (monocytes, granulocytes, and lymphocytes) and immunologic marker analyses CD4+T-lymphocytes, CD8+T-lymphocytes, activated T-lymphocytes (HLA-DR+CD3+), as well as T cell receptor (TcR) αβ+, TcRγδ+, CD4+CD45RA+and CD4+CD45RO+T-lymphocytes, B-lymphocytes (CD19+and/or CD20+) and NK cells (CD16+and/or CD56+/CD3−) in cord blood and venous blood at 3 and 18 mo of age were assessed in a subgroup of 55 infants. There was no relationship between pre-and postnatal PCB/dioxin exposure and upper or lower respiratory tract symptoms or humoral antibody production. A higher prenatal PCB/ dioxin exposure was associated with an increase in the number of TcRγδ+T cells at birth and with an increase in the total number of T cells and the number of CD8+(cytotoxic), TcRαβ+, and TcRγδ+T cells at 18 mo of age. A higher prenatal as well as postnatal PCB/dioxin exposure was associated with lower monocyte and granulocyte counts at 3 mo of age. In conclusion, our study suggests that background levels of PCB/dioxin exposure influences the human fetal and neonatal immune system.