Information about autoimmune diseases in cohort members and their mothers, fathers, and siblings was obtained from the Danish National Hospital Register, which contains information on all discharges from Danish hospitals since 1977; outpatients have been included in the register since 1995 (28). Diagnostic information in the National Hospital Register is based on the Danish version of the ICD-8 from 1977 to 1993, and the ICD-10 from 1994 to 2006.