Pneumocystis carinii pneumonia (PCP) is an opportunistic infection that occurs in immunosuppressed populations, primarily patients with advanced human immunodeficiency virus infection. The classic presentation of nonproductive cough, shortness of breath, fever, bilateral interstitial infiltrates and hypoxemia does not always appear. Diagnostic methods of choice include sputum induction and bronchoalveolar lavage. The drug of choice for treatment and prophylaxis is trimethoprim-sulfamethoxazole, but alternatives are often needed because of adverse effects or, less commonly, treatment failure. Adjunctive corticosteroid therapy improves survival in moderate to severe cases. Complications such as pneumothorax and respiratory failure portend poorer survival. Prophylaxis dramatically lowers the risk of disease in susceptible populations. Although PCP has declined in incidence in the developed world as a result of prophylaxis and effective antiretroviral therapy, its diagnosis and treatment remain challenging.