Chemically processed, hydrogenated fats, such as margarine, have been implicated in the aetiology of inflammatory bowel disease. Toxic by-products may occur in their production or during frying and cooking. A survey of dietary oil usage was conducted among Asians in Leicester, comparing inflammatory bowel disease patients with healthy controls. Two groups were comparable for age, sex, religion, place of birth, number of years spent in Britain and vegetarian status. There were no significant differences in actual oils used between healthy controls and patients with ulcerative colitis or Crohn's disease (chi 2 = 0.142 and 1.803 respectively, p greater than 0.50). However patients with Crohn's disease were found to recycle their cooking oil significantly more often than age and sex-matched colitics (p less than 0.05) and particularly age and sex-matched controls (p less than 0.01). A similar study needs to be conducted in India where the incidence of inflammatory bowel disease is low. If this difference is confirmed a programme of health education in cooking habits could lower the incidence of Crohn's disease.