In vivo evidence suggests that interleukin-18 (IL-18) shapes the development of adaptive immunity toward T-helper cell type 1 (Th1) responses. Monocyte-derived dendritic cells 1 (DC1s) preferentially induce a Th1 response, while plasmacytoid DC-derived DC2s have been linked to a Th2 response. We analyzed the role of IL-18 during the initiation phase of a Th response in vitro to elucidate the basis of these in vivo observations. IL-18 was constitutively released from DC1s, but not DC2s. Neutralization of IL-18 in coculture experiments of DC1s with allogeneic naive T lymphocytes did not alter the Th1/Th2 phenotype, while anti-IL-12 efficiently down-regulated the Th1 response. Unexpectedly, IL-18 receptor (IL-18R) alpha and beta chains were expressed on DC2 lineage. IL-18R expression was functional, as IL-18 induced chemotaxis in plasmacytoid DCs (pre-DC2s) and enhanced the allostimulatory capacity of IL-3-differentiated DC2s. Pre-DC2s exposed to IL-18 skewed the development of Th cells toward Th1 in coculture experiments of DC2s and allogeneic naive T cells, which was inhibited by IL-12 p70 neutralization. IL-18 might have a profound role during the initiation phase of an immune response by recruiting pre-DC2s and modulating the function of DC2s.