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.