The transcription factor nuclear factor kappaB (NF-kappaB) regulates the expression of inducible nitric oxide synthase (iNOS). We hypothesized that induction of iNOS in Helicobacter pylori gastritis may be due to NF-kappaB activation. Antral biopsy specimens from Helicobacter pylori-infected gastritis patients were collected before (n = 30) and after antimicrobial therapy to clear the infection (n = 22). Biopsies were assessed for NF-kappaB by immunohistochemistry (p65). The mRNA and protein of iNOS were localized by in situ RT-PCR and immunohistochemistry. Both of iNOS protein and mRNA were evident in stromal inflammatory cells, but absent in epithelia. Antimicrobial therapy resulted in a 73% reduction in iNOS levels (protein, p <.002). Nuclear staining for NF-kappaB p65 was evident in epithelial cells, especially in the neck region of gastric glands, and inflammatory cells. Treatment to clear H. pylori infection resulted in a 74% reduction in the epithelial staining for NF-kappaB p65 (p =.0001), whereas the lamina propria staining was unaltered. In conclusion, H. pylori infection activates NF-kappaB and iNOS expression. However, as the changes in NF-kappaB and iNOS with H. pylori clearance occurred in different cell types (epithelial vs. inflammatory), it appears that a NF-kappaB-dependent epithelial-derived mediator may be responsible for the induction of iNOS expression.