The onset of Crohn's disease (CD) is more frequent between 15 and 30 years and also has a second peak over 60 years. It female sex it is slightly more prevalent among female. Incidence rates vary in different geographic areas, ranging from 2 to 4 new cases/100,000 inhabitants/year. Prevalence data range from 1.2 to 200,000/100,000. Ethnic differences are beginning to disappear, underlining the role of environmental factors in the genesis of the disease. At the moment the most reliable hypothesis is that CD has a multifactorial pathogenesis: antigenic and environmental factors acting in genetically predisposed patients. According to this hypothesis, first degree relatives of CD patients have a relative risk of disease ranging from 2 to 4. Many infective agents have been associated with its onset and relapse; the most reliable observations are those related to the measles virus. There is an activation of the immunosystem which is shown by abnormal T-cell activation, by the increase of some cytokines and by the expression of adhesion molecules. The role of oral contraceptives and diet as risk factors is controversial. Smoking is the only risk factor positively associated with the onset of CD and its clinical course; this association is specific and dose-related.