Throughout the epidemic, Pneumocystis carinii pneumonia (PCP) has been the most common AIDS-defining opportunistic infection in the United States. With the widespread use of highly active antiretroviral therapy (HAART) and prophylaxis in patients known to be at risk, the incidence of PCP in patients with AIDS has declined dramatically. However, it is still seen regularly in patients with previously undiagnosed human immunodeficiency virus (HIV) infection, those who do not comply with prophylactic medications, and in occasional cases of failure of prophylaxis. Despite many years of study, our understanding of the biology, ecology, and pathogenesis of PCP is inadequate. Clinically, PCP in AIDS tends to be a less acute and milder illness than PCP in other types of immunocompromised hosts. Although the radiograph typically shows bilateral diffuse granular opacities, many other patterns are seen. Trimethoprim-sulfamethoxazole is the preferred drug for treating and preventing PCP, but toxicity limits its use. The choice of treatment is influenced by the severity of illness and relative toxicities of antipneumocystis agents. Adjunctive corticosteroid therapy is recommended for patients with moderate or severe disease. The success of HAART has prompted investigators to question whether prophylaxis against PCP and other opportunistic infections is necessary in patients who respond with a rise in CD4 lymphocyte counts and suppression of HIV replication.