Genetic factors have been implicated in the etiology of inflammatory bowel disease (IBD) because of the increased occurrence of IBD in relatives. To further characterize the familial aggregation of IBD, we obtained family histories by interview on 188 IBD patients, including 154 Ashkenazi Jews (82%), ascertained through a Los Angeles gastroenterology practice. Thirty-three index cases (17.6%) had at least one affected first-degree relative; an additional 11 had more distant affected relatives. Thus, 23.4% of our sample had a positive family history. The quantification of empiric risk estimates for various classes of relatives has been quite limited and has been reported in only a few series. An important goal of our study was the determination of the specific empiric risk figures for relatives. We obtained uncorrected risk estimates of 2.5% to off-spring, 5.2% to siblings, and 2.9% to parents. Although the highest risk we observed is to siblings, IBD has a variable and often late age of onset, and it is likely that many relatives, particularly offspring, of patients in this sample have not reached the age at which they will manifest clinical disease. Thus, these uncorrected risks as well as those reported in the literature are an underestimate of the true empiric risks. To provide an estimate of the true lifetime risks, we utilized age-specific incidence data to calculate the following age-corrected empiric risk estimates for IBD: 8.9% to offspring, 8.8% to siblings, and 3.5% to parents. It is these latter age-corrected estimates that are most appropriate for both genetic counseling and genetic modeling.