With a multistage laboratory protocol immunodeficiencies can be efficiently identified. The article presents a diagnostic protocol that consists of three schemes. Scheme 1 describes the diagnostic protocol for the large group of patients with recurrent pulmonary and ENT-infections, where an antibody deficiency can occasionally be found. Scheme 2 presents the diagnostic protocol for the much smaller group of patients with opportunistic infections, wasting or failure to thrive. Several of these patients suffer from a severe T-lymphocyte disorder. Early diagnosis and treatment is important for the prognosis in these patients. Scheme 3 shows the diagnostic protocol for patients with recurrent infections of surface areas and deeper organs; these patients may suffer from a phagocyte disorder.