The present study investigated how often anxiety disorders with different ages of onset occurred in affected families compared to control families. Furthermore, the study addressed the impact of sex, region of residence, year and month of birth, and parental age at birth. The sample included N=1373 child and adolescent psychiatric participants born between 1952 and 2000 and registered in the Danish Psychiatric Central Register (DPCR) who developed an anxiety disorder before the age of 18. N =4019 controls without any psychiatric diagnosis before age 18, were matched for age, sex, and residential region. Psychiatric diagnoses were also obtained for parents, siblings, and offspring. A family load component was obtained by using various mixed regression models. Anxiety disorders occurred significantly more often in case than in control families. Having a mother, father, or a sibling with the disorder was proven to be a risk factor. Female sex, year of birth, and region of residence were also associated with having an anxiety disorder. Furthermore, case relatives did not develop an anxiety disorder earlier than control relatives. These findings, based on a very large and representative dataset, provide further and solid evidence for the family aggregation of anxiety disorders.