In a case-control study the effects of smoking and the use of oral contraceptives were examined in 83 patients with Crohn's disease and 37 patients with ulcerative colitis in comparison with members of the general population who served as controls. After controlling for oral contraceptives, smokers were found to have a significantly elevated relative risk of contracting Crohn's disease of 3.0 (1.3 to 6.8). Patients on oral contraceptives for more than three years had an elevated risk for Crohn's disease of 4.3 (1.3 to 14.4); this elevated risk was, however, found only in the non-smoking group. In the case of ulcerative colitis, the relative risk in smokers was significantly lower (0.21 [0.04 to 0.91]) than in non-smokers. The use of oral contraceptives does not represent a risk factor for ulcerative colitis. While smoking has been established as a risk factor in the development of inflammatory bowel disease, the role of oral contraceptives in the aetiology of this condition remains unclear.