The mechanism by which mutations in CARD15, which encodes nucleotide-binding oligomerization domain 2 (NOD2), cause Crohn disease is poorly understood. Because signaling via mutated NOD2 proteins leads to defective activation of the transcription factor NF-kappa B, one proposal is that mutations cause deficient NF-kappa B-dependent T helper type 1 (T(H)1) responses and increased susceptibility to infection. However, this idea is inconsistent with the increased T(H)1 responses characteristic of Crohn disease. Here we used Card15(-/-) mice to show that intact NOD2 signaling inhibited Toll-like receptor 2-driven activation of NF-kappa B, particularly of the NF-kappa B subunit c-Rel. Moreover, NOD2 deficiency or the presence of a Crohn disease-like Card15 mutation increased Toll-like receptor 2-mediated activation of NF-kappa B-c-Rel, and T(H)1 responses were enhanced. Thus, CARD15 mutations may lead to disease by causing excessive T(H)1 responses.