Damage to the immune system induced by the human immunodeficiency virus (HIV) leads to a spectrum of opportunistic infections of which the lung is the most common site. In Europe and North America, pneumocystis carinii pneumonia is the presenting symptom in 64% of cases of acquired immunodeficiency syndrome (AIDS) and occurs at some point in 80% of AIDS victims. This infection is less common in Africa, where tuberculosis is the predominant opportunistic infection. Other AIDS-related lung infections that are gaining in prevalence include pneumonia due to pyogenic bacteria, pulmonary infection with Mycobacterium tuberculosis, and lymphoid interstitial pneumonitis. In addition, there is evidence that the lung may be extensively involved in Kaposi's sarcoma. Given the importance of the lung as a site for AIDS-related opportunistic infections, respiratory physicians will be required to become more involved in the diagnosis and management of AIDS cases.