One of the few aspects of the epidemiology of inflammatory bowel disease (IBD) that seems indisputable is the emergence of both ulcerative colitis and Crohn’s disease in the economically developed nations of Western Europe and North America during the past century.1 Some of the apparent increase in incidence undoubtedly reflects a receding tide of mimicking infectious diseases and developments in endoscopic and other diagnostic techniques. Nevertheless, most epidemiologists accept that there were considerable real increases in ulcerative colitis incidence during the first half of this century and of Crohn’s disease incidence during the middle half-century in these countries.2 As the millennium approaches, are the incidences of each disease continuing to increase? Some reports suggest they are, whereas claims that Crohn’s disease incidence is no longer increasing can provoke a rapid response from some clinicians.3-6 At a time when basic science is making progress by leaps and bounds, why should such an apparently simple epidemiological question prove so difficult to answer, and does it matter?

Assessing time trends in the incidence of most chronic diseases is difficult and IBD poses particular problems. Firstly, the data routinely collected on mortality and morbidity from IBD are of little help. In most countries over 75% of deaths attributed to IBD are in the elderly and mortality rates are a twentieth or less of the incidence figures for the same period. Although national mortality statistics show a broad correlation with individual incidence studies from those countries, the relation between mortality and incidence within countries is weak, such that during the 1950s and 1960s mortality rates from Crohn’s disease in England and Wales and the USA showed a small less than twofold increase when individual studies of Crohn’s disease incidence were showing much larger increases.7 ,8 Over the same period ulcerative …