Contact with chemically modified, especially hardened, fats was found in all of 90 patients with Crohn's disease of the small or large intestine. These fats (margarine, frying or cooking fat) had been encountered before or at commencement of the disease and were used exclusively for cooking in 42 cases, additionally as bread spreads in 46 cases and as bread spread only in 2 cases. Communal meals were taken by 89 patients, in 55 of these at least once per week. 13 patients with anorectal manifestation of Crohn's disease had contact with hardened fats as rectal suppositories prior to or at commencement of the disease. These findings are compatible with the concept that chemically processed fats may play a role in the development of Crohn's disease in susceptible individuals.