To test the hypothesis that environmental factors play a role in idiopathic inflammatory bowel disease (IBD), age, period, and generation (cohort) effects on IBD mortality in the United Kingdom and the United States were examined. The crude death rate of ulcerative colitis has declined since 1930. Plotted versus the year of birth, its age-specific death rates showed an initial rise in successive generations born between 1850 and 1900, followed by a fall in all later generations. The crude death rate of Crohn's disease increased from 1950 to 1974 and then declined. When the age-specific death rates for Crohn's disease and ulcerative colitis were superimposed, the mortality from Crohn's disease in each age group or sex began to decline at a different time, but always upon reaching the level of mortality from ulcerative colitis. It appears as if the gradual disappearance of an ulcerative colitis-associated factor in a birth-cohort fashion prevented a further rise in mortality from Crohn's disease after 1974.