Data indicate that appendicectomy for intra-abdominal inflammation protects against inflammatory bowel disease (IBD). This suggests an important role for the appendix in mucosal immunity. There is no established model of appendicitis. We therefore developed a murine model of appendicitis and examined the effect of inflammation on appendiceal lymphocyte constituents. The caecal patch of specific pathogen-free (SPF)-Balb/c mice was transformed into an obstructed 'appendiceal pouch' by standardized suction and band ligation. Mice were killed and 'pouches' removed for histology and phenotypic analysis of leucocytes by flow cytometry. Serum C-reactive protein (CRP) was determined by enzyme-linked immunosorbent assay. All 'pouches' developed features resembling human appendicitis - mucosal ulceration, transmural inflammation with neutrophils, lymphocytes and occasional eosinophils, and serositis. These changes were most evident between days 7 and 10. There was significant elevation of serum CRP (8.0 +/- 0.3 ng/ml to 40.0 +/- 3.1 ng/ml; P < 0.01), indicating systemic inflammation. Following the initial neutrophil-predominant response, there was an increase in CD4(+) (15.3% +/- 1.2% to 31.0 +/- 2.0%; P < 0.01) and CD8(+) T lymphocytes (3.7% +/- 0.6% to 9.2 +/- 0.8%; P < 0.01). CD25(+) forkhead box P3 (FoxP3)(+) regulatory T lymphocytes were increased by 66% (P < 0.01). Furthermore, significant increases in CD8(+) FoxP3(+) regulatory T lymphocytes were restricted to younger mice (age < 10 weeks, P < 0.003). This is the first description of a murine model of appendicitis. Inflammation resulted in T lymphocyte accumulation associated with an increase in regulatory T lymphocytes, which might explain the age-dependent protective phenomenon. Further exploration will provide insights into the mechanisms of intestinal immune homeostasis and the immunopathogenesis of IBD.