To assess the familial aggregation of inflammatory bowel disease (IBD) in Italy, the family pedigrees of 411 patients with ulcerative colitis (UC) and 241 patients with Crohn's disease (CD) seen at 14 participating hospitals were studied. Sufficient information was obtained on 97% of 3752 first-degree relatives, 80% of 8869 second-degree relatives, and 74% of 5791 cousins. Thirty-six propositi (5.52%) had a total of 44 affected relatives (16 CD, 28 UC). The prevalence of IBD was higher in first- than in second-degree relatives and cousins (791, 112, and 163 in 100,000, respectively). A strong intrafamilial disease concordance was observed, with 26 cases of UC and 6 of CD among relatives of UC patients and 10 cases of CD and 2 of UC among relatives of CD patients. The prevalence of UC among first-degree relatives of UC patients and that of CD among first-degree relatives of CD patients was 680 and 531 in 100,000, respectively. In conclusion, there is a high degree of familial aggregation for IBD in Italy, with a strong intrafamilial disease concordance.