BACKGROUND Expression of pro-inflammatory cytokines is increased in the intestinal lamina propria of patients with inflammatory bowel disease (IBD). Nuclear factor kappa B (NF kappa B) controls transcription of inflammation genes. On activation, NF kappa B is rapidly released from its cytoplasmic inhibitor (I kappa B), transmigrates into the nucleus, and binds to DNA response elements in gene promoter regions. OBJECTIVE To investigate whether increased activation of NF kappa B is important in IBD and may be down-regulated by anti-inflammatory treatment. METHODS Activation of NF kappa B was determined by western blot assessment and electrophoretic mobility shift assay in nuclear extracts of colonic biopsy samples as well as lamina propria mononuclear cells. RESULTS Nuclear levels of NF kappa B p65 are increased in lamina propria biopsy specimens from patients with Crohn's disease in comparison with patients with ulcerative colitis and controls. Increased activation of NF kappa B was detected in lamina propria mononuclear cells from patients with active IBD. Corticosteroids strongly inhibit intestinal NF kappa B activation in IBD in vivo and in vitro by stabilising the cytosolic inhibitor I kappa B alpha against activation induced degradation. CONCLUSIONS In both IBDs, but particularly Crohn's disease, increased activation of NF kappa B may be involved in the regulation of the inflammatory response. Inhibition of NF kappa B activation may represent a mechanism by which steroids exert an anti-inflammatory effect in IBD.