Aims: This study used nationwide hospital records to examine sibling risk of any type of anxiety disorder in Sweden over a 40-year period. Methods: This study, carried out between 1 January 1968 and 31 December 2007, of the entire population of Sweden, linked information on family relationships from the nationwide Multi-Generation Register with information from the nationwide Swedish Hospital Discharge Register on first diagnosis of anxiety disorder. A total of 42 602 persons hospitalized for anxiety disorders and 2093 affected siblings were identified. Standardized incidence ratios (SIR) were calculated by comparing risk in siblings of persons hospitalized for anxiety disorders with risk in persons whose siblings had no hospital diagnosis of anxiety disorders. Results: The sibling risk was 2.26, which was independent of sex and age differences between siblings. The SIR was highest in siblings <20 years of age (2.83). Analysis of risk by subtype showed that having a sibling diagnosed with any anxiety disorder resulted in increased risks of a number of disorders; the highest increased risk was of social phobia (SIR 3.68, 95% confidence interval, 1.68-7.69). Risk of panic disorder, generalized anxiety disorder, mixed anxiety and depressive disorder, and obsessive-compulsive disorder was raised in female but not male siblings. Conclusions: Heritable effects likely play an important role in the cause of anxiety disorders, but the extent of their role remains to be established. Important contributions could be made by studies of gene-environment interactions that have sufficient sample sizes to produce reliable results.