[Genetic aspects of Crohn disease and ulcerative colitis. Empirical risk of recurrence and case presentation].

During the last decades Morbus Crohn and ulcerative Colitis, both chronic inflammatory bowel diseases, show increasing incidence rates. The etiology of the diseases remains unclear. The well known familial occurrence of these diseases will create an increasing demand for medical, psychological and genetic counselling in face of the increasing incidence. The recurrence risks for close relatives will be presented and discussed. An example of Crohn's disease in two twin-siblings illustrates the familial occurrence and underlines the usefulness of being aware of possible recurrence in close relatives.