Abstract Bone marrow-derived plasmacytoid dendritic cells (pDCs) from IL-1R–associated kinase (IRAK)2-deficient mice produced more IFNs than did wild-type pDCs upon stimulation with the TLR9 ligand CpG. Furthermore, in CpG-stimulated IRAK2-deficient pDCs there was increased nuclear translocation of IFN regulatory factor 7, the key transcription factor for IFN gene transcription in these cells. In IRAK2-deficient macrophages, enhanced NF-κB activation and increased expression of CpG-induced genes were detected within 2 h after treatment. However, at later times, NF-κB activation was decreased and, in contrast to the results with IFN, there was less secretion of other proinflammatory cytokines (such as TNF-α) and chemokines in CpG-stimulated IRAK2-deficient pDCs and macrophages. Therefore, although IRAK2 is a negative regulator of TLR9-mediated IFN production through its modulation of the transcriptional activity of IFN regulatory factor 7, it is also a positive regulator of TLR9-mediated proinflammatory cytokine and chemokine production at some level subsequent to transcription.