An inverse association between appendicectomy and risk of ulcerative colitis was first reported in 1987 as an unexpected finding in a study of childhood determinants of inflammatory bowel diseases.1 The major impetus, however, for the current interest in the association was a 1994 case-control study which reported that only 0.6% of patients with ulcerative colitis had had their appendix removed before diagnosis, compared with 25.4% of controls from orthopaedic clinics. The corresponding odds ratio of 0.02 launched the idea that appendicectomy protects against ulcerative colitis.2 But is this so?

Subsequent case-control studies have confirmed the inverse association, although with considerably less extreme odds ratios. In 2001, however, review of the available literature showed that most studies had used inappropriate or questionable methods,3 and the number of unclear or overtly flawed case-control studies has increased since then. One common problem is the failure to use identical methods and periods for the ascertainment of previous appendicectomies in the patients with ulcerative colitis and in …