Objective. - Clinical care of people infected with human immunodeficiency virus (HIV) has been substantially affected by the introduction of potent antiretroviral therapy. Changes in the immune system after such therapy and the clinical consequences are important issues for clinicians treating patients with HIV. Data Sources. - A systematic review of MEDLINE, 1993 to January 1998, of peer-reviewed publications, abstracts from national and international conferences, and product registration information through January 1998. Study Selection and Data Extraction. - Criteria used to select studies include relevance to immune reconstitution with potent antiretroviral therapy and having been published in the English language. Assessment of data quality and validity included consideration of venue of the publication and relevance to practice. Data Synthesis. - Suppression of viral replication after administration of potent antiretroviral therapy that includes inhibitors of the HIV-1 protease is associated with quantitative and qualitative changes in the immune system. In patients with relatively advanced disease, there is a first-phase rise (during the initial 3 months) in both naive and memory CD4+ and CD8+ T lymphocytes and B lymphocytes. This is followed by a slower second-phase increase (after 3 months) in cells primarily of the naive CD4+ and CD8+ phenotypes. These quantitative changes are associated with qualitative improvements in host immune responses, best characterized by dramatically reduced risk of opportunistic infection. Restoration of the immune system during the first year of potent antiretroviral therapy is partial at best. Conclusions. - Potent antiretroviral therapy has become the standard of care for people with HIV infection, and its use has led to significant reductions in the incidence of the acquired immunodeficiency syndrome (AIDS) and in mortality from HIV infection. Although incomplete, considerable immune recovery occurs, sufficient, in most cases, to provide adequate protection against most AIDS- associated opportunistic infections.