This study represents the first national epidemiological survey of primary immunodeficiency diseases in Norway. Uniform questionnaires were sent out in April 1998 to all hospital departments considered relevant. As of February 1999, a total of 372 patients have been registered, of whom 69 patients are deceased. With a population of 4.45 million people, the total prevalence of primary immunodeficiency diseases in Norway February 1, 1999 is 6.82 per 100000 inhabitants. Distribution between the main immunodeficiency diagnoses is (a) antibody deficiencies 50.8%, (b) combined deficiencies included other immunodeficiency syndromes 12.4%, (c) complement deficiencies 21.0%, (d) phagocytic disorders 6.7%, (e) and immunodeficiency associated with other congenital diseases 9. 1%. Compared to previous reports from other European countries, there is a smaller proportion of antibody deficiencies due to few IgA deficiencies registered and a large proportion of complement deficiencies due to many patients with hereditary angioedema.