Toll-like receptors (TLRs) sense microbial products and initiate adaptive immune responses by activating dendritic cells (DCs). As pathogens may contain several TLR agonists, we sought to determine whether different TLRs cooperate in DC activation. In human and mouse DCs, TLR3 and TLR4 potently acted in synergy with TLR7, TLR8 and TLR9 in the induction of a selected set of genes. Synergic TLR stimulation increased production of interleukins 12 and 23 and increased the Delta-4/Jagged-1 ratio, leading to DCs with enhanced and sustained T helper type 1-polarizing capacity. Global gene transcriptional analysis showed that TLR synergy 'boosted' only approximately 1% of the transcripts induced by single TLR agonists. These results identify a 'combinatorial code' by which DCs discriminate pathogens and suggest new strategies for promoting T helper type 1 responses.