We conducted a study to examine the relation of asthma and its treatment to irritable bowel syndrome (IBS). We identified cohorts of members with and without asthma from January 1996 though June 2002 and calculated rates of IBS in both cohorts. In a nested case-control study, we calculated odds ratios for oral steroid dispensing before IBS onset. To address differential IBS detection, we conducted a quantitative sensitivity analysis. We identified 91,237 people with asthma and sample of 24,518 people without asthma. There was a 20% increase in the incidence of IBS among people with asthma (standardized morbidity ratio = 1.2; 95% CI, 1.0-1.5), and no association between oral steroids and IBS among people with asthma (OR = 1.0; 95% CI, 0.9-1.1.) Misclassification of IBS would have biased the rate ratio toward the null. We observed a small increase in IBS among people with asthma, and no association between oral steroids and IBS onset among asthma patients.