There are strong indicators that the aetiology of inflammatory bowel disease should be regarded as multifactorial, involving an interaction between genetic and environmental factors which give rise to an inadequate immunological response. During the past decade at least seven case-control studies have shown an inverse association between appendectomy and ulcerative colitis. Conclusions have been that either ulcerative colitis protects against appendicitis, or appendectomy protects against ulcerative colitis. The immunological function of the appendix is not well known, but experimental studies suggest that the appendix is possibly an important site for priming of the cells involved in the development of inflammatory bowel disease. Experimental and prospective cohort studies are needed to provide more insight in a possible relation between ulcerative colitis and the appendix.