Dietary hypersensitivity and inflammatory bowel disease (IBD) are important causes of chronic vomiting and diarrhoea in cats. In this study, the expression of genes encoding Il12p35 and p40, IL-23p19 and IFN- was studied in duodenal biopsies taken from control cats and cats with histological evidence of IBD. There were significant differences in expression of mRNA encoding IL-12p35 and IL-23p19 between healthy cats and cats with gastrointestinal disease. IL-12p35 mRNA was significantly lower in the duodenal mucosa of cats with signs of GI disease compared with the mucosa of healthy cats (p=0.000). In contrast, IL-23p19 mRNA expression was significantly higher in duodenal biopsies from cats with signs of GI disease than healthy controls (p=0.001). There was no significant difference in expression of IL-12p40 and IFN- mRNA (p>0.05). Histopathological scoring of duodenal sections was not significantly different between the two groups (p.0.05) and did not correlate with the levels of mRNA expression for any cytokine. The majority of cats with signs of GI disease had moderate to severe colitis (score 2), even those cats without signs of large bowel disease. The results of this study suggest that IL-23, and therefore the Th17 subset of CD4+ T cells, plays a role in the pathogenesis of feline IBD.