Research on inflammatory bowel disease risk among oral contraceptive users has reached conflicting conclusions. This population-based case-control study evaluated the effects of oral contraceptive use on ulcerative colitis and Crohn's disease risk. Cases were women enrollees, aged 15-68 years, of Group Health Cooperative of Puget Sound (a prepaid health plan based in western Washington State) who had ulcerative colitis (n = 211) or Crohn's disease (n = 91). Age-matched controls were randomly selected from the health plan enrollment file. An in-person interview obtained information about lifetime contraceptive use. Conditional logistic regression analysis was used to estimate relative risks and 95% confidence intervals for disease. Women who reported oral contraceptive use within 6 months before disease onset were at increased risk for both diseases compared with never users (relative risk (RR) of ulcerative colitis = 2.0, 95% confidence interval (CI) 1.2-3.3; RR of Crohn's disease = 2.6, 95% CI 1.2-5.5). Women who had used oral contraceptives for more than 6 years had the highest risk of Crohn's disease (RR = 5.1, 95% CI 1.8-14.3). In contrast, increasing duration of use was not associated with increased risk of ulcerative colitis. Adjustment for race, smoking, income, or pregnancy history did not substantially alter these results. Higher ulcerative colitis risk tended to occur among users of high estrogen dose oral contraceptives, while Crohn's disease risk was similar regardless of estrogen potency.