Inflammatory bowel diseases (IBDs) have been reported throughout the world, but their frequency is highest in Northern Europe and the USA. The mean incidence of ulcerative colitis (UC) is 6/10(5) inhabitants, and that of Crohn's disease (CD) is 4/10(5) inhabitants. IBDs occur in young adults, especially Crohn's disease which reaches a peak of incidence between the ages of 20 and 30 years. Among possible factors influencing their incidence, only tobacco smoking has been shown to facilitate the acute episodes and relapses of CD and to diminish those of UC. The frequency of familial IBDs, which mainly affects first degree relatives, varies from 6.1 to 35.8%. A genetic component is present and probably more important for CD than for UC, although no genetic marker has yet been identified.