A prospective epidemiological study on inflammatory bowel diseases (IBD) patients was performed in Brussels' area from April 1st, 1992 to 30th March, 1993. The mean annual incidence for Crohn's diseases (CD) was 4.1/10(5) inhabitants/year among native Belgian people and 6.4/10(5) inhabitants/year for subjects issued from Moroccan families. For ulcerative colitis, the incidence was 3.7/10(5) inhabitants/year for native Belgian people and only 1.2/10(5) inhabitants/year for Moroccan subjects. The male/female sex ratio was 0.4 for CD and 1.9 for UC. At the time of diagnosis, the mean age was 34 years for CD and 38 years for UC. For both diseases, the age peak was between 20 and 29 years. Cigarette smoking was significantly higher in CD (48%) than in UC patients (12%). Family history was about 10% for both diseases.