Prevalence of inflammatory bowel disease among relatives of patients with ulcerative colitis.

The familial occurrence of inflammatory bowel disease (IBD) was investigated among 963 patients with ulcerative colitis (UC) diagnosed in 1955-1979 in Stockholm County. For 76 patients who had a relative with IBD a pedigree was drawn. The diagnoses of the diseased relatives were verified. There was a general prevalence of 7.9% for IBD among relatives. In 80% one relative was affected, in most cases a first-degree relative with UC. Sibship was the commonest relationship. No concordance for UC was found among three pairs of monozygotic twins. The prevalence of UC in first-degree relatives was 15 times higher than in non-relatives. The age of onset was significantly lower among patients with a family history for UC; they also had a higher incidence of total colitis. The prevalence of Crohn's disease in first-degree relatives of patients with UC was almost 3.5 times higher than in non-relatives.