The lungs are the principal target organ in the infectious complications of acquired immunodeficiency syndrome (AIDS) and this predisposition to infections is not the regional manifestation of systemic immunologic deficiency induced by human immunodeficiency virus (HIV) only, because HIV also affects lung's own complex system of local defense mechanisms. It was demonstrated that pulmonary host defenses were compromised by.direct infection of alveolar macrophages with HIV and decreased production of solubile factors by lymphocytes derived from bronchoalveolar lavage fluid was shown. The most common infectious causative agents are facultative intracellular pathogens including Pneumocystis carinii, Cryptococcus neoformans, Mycobacterium tuberculosis and cytomegalovirus, which reflects the specific defects of cell-mediated immunity. AIDS patints have, in addition, an increased incidence of infections with capsulated bacteria such as Haemophilus influenzae and Sterptococcus pneumoniae which are associated typically with the impairment of the humoral immune response. High-grade pathogens such as M. tuberculosis tende to reactivate early in the progression of immunodeficiency whereas low-grade pathogens such as P. carinii only emerge when the defect is more advanced. The profound immunodeficiency in AIDS patients means that clinical features may be quite atypical and blunted.