The prevalence of familial inflammatory bowel disease was 13.4% in a population-based study of 1048 patients with Crohn's disease (CD). Seventy-two of the index cases had 82 first-degree relatives. Forty-nine were more distantly related (19 first cousins from the same generation as the index patient). The prevalence of CD among first-degree relatives was 21 times higher than among non-relatives. Four of six monozygotic twins were concordant. The age at onset was 25 years in the patients with familial CD, compared with 33 years in the entire group. An additional 53 relatives were found to have ulcerative colitis (UC). The prevalence of UC among first-degree relatives of patients with CD was six times higher than among non-relatives.