3,3'-Diindolylmethane (DIM) is a major acid-condensation product of indole-3-carbinol and is present in cruciferous vegetables. In this study, we evaluated the effects of DIM on antiinflammatory and antitumor promotion activity in mouse skin and explored the relevant mechanisms. When 12-O-tetradecanoylphorbol-13-acetate (TPA) was applied topically to the mouse ear to induce inflammation, DIM pretreatment effectively inhibited TPA-induced ear edema formation. To evaluate the mechanisms underlying DIM's antiinflammatory effects, DIM was topically treated to the shaved backs of mice 30 min before TPA treatment. DIM inhibited the TPA-induced increases in the expression of cyclooxygenase (COX)-2, inducible nitric oxide synthase (iNOS), chemokine (C-X-C motif) ligand (CXCL) 5, and interleukin (IL)-6 in mouse skin. DIM also inhibited nuclear factor-kappa B (NF-kappaB)'s DNA binding activity, the nuclear translocation of p65, and the degradation of inhibitor of kappaB (IkappaB) alpha in TPA-stimulated mouse skin. Furthermore, DIM reduced TPA-induced increases in the activity of extracellular signal regulated protein kinase (ERK)-1/2 and IkappaB kinase (IKK). When mouse skin papillomas were initiated via the topical application of 7,12-dimethylbenz[alpha]anthracene (DMBA) and promoted with repeated topical applications of TPA, repeated topical applications of DIM prior to each TPA treatment significantly suppressed the incidence and multiplicity of the papillomas. DIM also reduced the expression of COX-2 and iNOS, ERK phosphorylation, and the nuclear translocation of p65 in papillomas. Collectively, these results show that DIM exerts antiinflammatory and chemopreventive effects in mouse skin via the downregulation of COX-2, iNOS, CXCL5, and IL-6 expression, which may be mediated by reductions in NF-kappaB activation.