BACKGROUND & AIMS: Because the mode of Crohn's disease inheritance is unknown, age-adjusted risk estimates and knowledge of disease characteristics will aid genetic counseling and modeling. The aim of this study is to determine the prevalence of familial occurrence of inflammatory bowel disease in first-degree relatives of patients with Crohn's disease and estimate their age-adjusted risks. It also evaluates agreement in disease characteristics between generations within families with a history of Crohn's disease. METHODS: Six hundred forty patients with Crohn's disease and 800 control subjects were questioned about the occurrence of inflammatory bowel disease in their first-degree relatives. Agreement for age at diagnosis, initial disease location, disease behavior, and number of bowel resections was determined in 68 families with two or more members affected and compared with data in 100 unrelated patients with Crohn's disease. RESULTS: Probands with Crohn's disease had a more frequent positive family history than controls. The age at diagnosis between probands with and without a positive family history was insignificant. Crude and age-adjusted risk elements were higher in relatives of patients, especially daughters, compared with those of controls. The age at diagnosis was older for parents than offspring but similar between siblings. Initial disease location was especially striking between siblings. CONCLUSIONS: This study confirms familial aggregation and a high degree of disease concordance in Crohn's disease. The age at diagnosis and initial disease location was especially strong within generations.