The aetiology of ulcerative colitis and Crohn’s disease remains an enigma. There is, however, a growing consensus that early events/exposure are of importance and constitute a “window of opportunity”. Both analytical and observational studies as well as animal models give credence to this hypothesis. Smoking has, until recently, been the only factor that has been consistently associated with an increased risk of Crohn’s disease and a decreased risk of ulcerative colitis. Appendicectomy has now been added to this very short list as there has been a consistent finding in several studies that this operation is associated with a decreased risk of ulcerative colitis.1 ,2 Duggan et al’s study (see page 494) confirms these results. Their findings for childhood hygiene are in accordance with previous studies where the availability of hot running water is …