The prevalence of Crohn's disease (79/10(5)), ulcerative colitis (389/10(5)) and appendicectomy (2751/10(5)) were measured in a survey of the Mormon church in Britain and Ireland. Details were obtained from 230 of the 342 branches in Britain which had a membership of 56,000 people although only 17,700 people were active members. The prevalence of Crohn's disease was comparable to that reported from various centres in Britain, but ulcerative colitis was significantly commoner amongst Mormons while the frequency of appendicectomy was significantly reduced.