Plasmacytoid dendritic cells (pDCs) respond to unmethylated cytosine-phosphate-guanosine (CpG) motifs present in bacterial DNA or unmethylated synthetic oligodeoxynucleotides (CpG). In order to assess the function of pDCs in human newborns, interferon-alpha (IFN-alpha) production induced by CpG 2216 and phenotypic maturation of pDCs in response to CpG 2006 were compared in cord blood and adult blood. We first observed that neonatal pDCs displayed decreased up-regulation of CD80, CD83, CD86, and CD40, whereas HLA-DR and CD54 up-regulation did not differ significantly between adults and neonates. We then found that the production of IFN-alpha in response to CpG was dramatically impaired in cord blood. This neonatal defect was detected both at protein and mRNA levels and was still present in blood of 4-day-old babies. Further experiments on enriched pDCs confirmed that these cells are intrinsically deficient in CpG-induced IFN-alpha production at birth. These findings might be relevant to the increased susceptibility of human newborns to infections as well as to the use of CpG oligodeoxynucleotides as vaccine adjuvants in the neonatal period.