To test the hypothesis that Crohn's disease is caused by delayed exposure to enteric infections, we did a case-control study. We compared 133 patients who have Crohn's disease and 231 with ulcerative colitis who have controls selected from the general population and matched for age and sex. Crohn's disease was more common in subjects whose first houses had a hot-water tap (odds ratio 5.0, 95% CI 1.4-17.3) and separate bathroom (3.3, 1.3-8.3). Ulcerative colitis showed no clear relation to household amenities in infancy. These findings may explain why the incidence of Crohn's disease has increased in developed countries over the past 50 years.