Many defense mechanisms are located in the respiratory tract, since they are constantly exposed to a variety of pathogens from both inside and outside the body. Among the various defense mechanisms, the immune system is so potent that immunodeficiency results in proneness to infections and repeated and/or intractable infection. It is not uncommon to encounter patients with congenital immunodeficiency in clinical practice because of recent advances in supportive therapy. This paper describes cases of primary immunodeficiency that developed respiratory infections, together with the results of investigations of their immune status. Doctors should suspect the possibility of immunodeficiency in patients with repeated or intractable respiratory infections. Intractable respiratory infections, however, have been seen with increasing frequency in the elderly because of the increase in the elderly population. One reasons for this is the physiologic immunodeficiency associated with aging, which is characterised by decreased antigen-induced lymphocyte proliferation, IL-2 production, IL-2R expression, hsp 70 mRNA production, and demethylation of T lymphocytes, and delayed degradation of c-myc mRNA.