The incidence and prevalence of inflammatory bowel disease were estimated in all children less than 16 years of age living in the city of Göteborg and in three countries in South-Western Sweden, from 1983 to 1987. One hundred and thirty-two patients were classified according to set criteria into one of four diagnostic categories: ulcerative colitis, Crohn's disease, probable Crohn's disease and indeterminate colitis. The crude incidence of inflammatory bowel disease was 5.3 per 100,000 children per year and the prevalence 21.5 per 100,000 children. This study lends support to the hypothesis that Crohn's disease has increased among Swedish children. Crohn's disease now appears to be at least as common as ulcerative colitis. Thirty-five of 55 patients first classified as indeterminate colitis or probable Crohn's disease later fulfilled the criteria of ulcerative colitis or Crohn's disease during a mean follow-up period of 4.6 years. This study emphasizes the importance, in epidemiological studies of inflammatory bowel disease, of including those cases where a definite diagnosis of ulcerative colitis or Crohn's disease cannot be established initially and of re-evaluating the initial diagnosis regularly.