A sharp rise in the incidence of inflammatory bowel disease (IBD) has been observed in the western world since the early 1950s. The increase in the incidence of ulcerative colitis preceded the increase in the incidence of Crohn's disease by about 10-15 years. In high-incidence areas, a female preponderance at a young age (20-40 years) is observed in Crohn's disease, whereas in ulcerative colitis male incidence is still high at older ages. IBD is more common in the developed world than in the developing world and, in both the United States and Europe, a north-south incidence gradient has been reported, with IBD more common in the north than in the south. There are also indications that, in typically low-incidence areas, more cases are being seen lately. At present, IBD is rather common in western Europe, affecting 0.5-1.0% of the population during their lifetime. It will be interesting to follow these temporal trends in the coming years, as they may teach us more about the role of environmental factors in the pathogenesis of IBD.