Dendritic cells (DCs) and mast cells (MCs) co-localize in peripheral tissues of antigen entry, i.e. skin and mucosa. Due to the proximity of these two cell types, activation of MCs may affect DC functions. Here, we co-cultured human monocyte-derived DCs with cord blood-derived MCs activated by cross-linking of FcepsilonRI to elucidate the net effect of the whole MC products on DCs. Activated MCs induced maturation of DCs, and potently suppressed IL-12p70 production by the DCs. Whereas co-culture of DCs with activated MCs alone did not significantly influence the type of CD4(+) T cell responses induced by the DCs, DCs co-cultured with activated MCs in the presence of pro-inflammatory or T(h)1-inducing factors caused T(h)2 polarization. Although histamine was involved in the induction of DC maturation and T(h)2 polarization by activated MCs, a combinatorial effect of various MC-derived factors, including those acting in a cell contact-dependent manner, was required for the optimal induction of T(h)2-promoting DCs. Furthermore, we demonstrated that clusters of DCs are located closely with MCs in lesions of atopic dermatitis. Collectively, this study suggests that the interaction between DCs and IgE-activated MCs in a pro-inflammatory or even T(h)1-prone environment is instrumental in maintaining and augmenting T(h)2 responses in allergy, and that disruption of the DC-MC interaction may constitute an effective strategy to treat ongoing allergic diseases.