Nitric oxide produced by inducible nitric oxide synthase (iNOS) has been claimed to be involved in gastritis; however, its characteristics are largely unknown. We assessed (1) iNOS expression in the human gastric mucosa in chronic gastritis and (2) the cytokines associated with iNOS expression. Gastric biopsy specimens were obtained from patients with chronic gastritis. Total RNA was isolated and reverse transcription-polymerase chain reaction (PCR) was performed semiquantitatively using specific primer sets for iNOS, interleukin (IL)-1beta, IL-8, IL-6, interferon-y, tumor necrosis factor-alpha (TNF-alpha), TNF receptors, IL-6 receptors, and sucrase, respectively. Helicobacter pylori infection was examined by the PCR assay. Reverse transcription-PCR analysis showed that expression of iNOS was detected in 10 of 23 samples. Expression of iNOS mRNA was closely correlated with expression of TNF-alpha and was observed frequently in subjects with intestinal metaplasia. The Helicobacter pylori gene was detected by PCR assay in all iNOS-positive cases. These results indicate that iNOS is predominantly expressed in the gastric mucosa with intestinal metaplasia and H. pylori infection. TNF-alpha is thought to be an important cytokine associated with iNOS expression.