Chronic inflammatory bowel diseases, ulcerative colitis and Crohn's disease, are steadily increasing in prevalence, and one half to one per cent of the Swedish population are currently estimated to be affected. The aetiology remains unknown, but is probably multifactorial. Both dietary, microbiological and immunological causes have been discussed. Clinical studies, including several Swedish studies, have also shown genetic factors to be crucially involved. Findings in sophisticated molecular biological studies suggest certain specific genes to be involved, and a current EU project in which Sweden is participating has been launched to map the mode of inheritance in detail.