Recent reports of reduced appendectomy rates in patients with ulcerative colitis have not distinguished between primary appendectomy (surgery for appendicitis) and incidental appendectomy (removal of the appendix for other reasons). In the present case control study, we examined the frequency of primary appendectomy in subjects with ulcerative colitis (n = 197) and Crohn's disease (n = 117) compared to a control group of dermatology outpatients (n = 243). A reduced rate of primary appendectomy was found in the ulcerative colitis group (adjusted odds ratio 0.20, 95% confidence intervals 0.070-0.53, p < 0.0005) but not in the Crohn's disease patients (adjusted odds ratio 0.93, 95% confidence intervals 0.39-2.18, p = NS). These data suggest that appendicitis occurs less commonly than would be expected in individuals who develop ulcerative colitis. Environmental or immunoregulatory factors may be responsible. Tonsillectomy rates were also examined in each study group, but no overall differences were found between patients with inflammatory bowel disease and controls.