In much of the world, pneumocystosis remains the most common life-threatening opportunistic infection among patients with HIV disease. The infection is caused by Pneumocystis carinii--an organism whose identity as a fungus or parasite is still debated. What is no longer debated, after a decade of AIDS, is that pneumocystosis is almost entirely preventable and eminently treatable. Understanding has improved concerning when prophylaxis should be initiated. It is also recognized that, at least with the agents available today, antiretroviral therapy alone will not prevent pneumocystosis. Sputum induction and the use of monoclonal antibodies have modestly improved our ability to diagnose the infection; however, invasive procedures are still required for most patients, and unusual presentations of the disease, such as cavitary lesions, apical infiltrates, pneumothoraces, and extrapulmonary infection, are not infrequently seen. For treatment, trimethoprim-sulfamethoxazole and intravenous pentamidine remain the mainstays; oral therapy with dapsone and trimethoprim can be as effective as conventional therapy in mild disease, permitting treatment on an outpatient basis. Adjunctive steroids are useful for treatment of moderate to severe pneumocystosis, but clinicians should be alert to the possibility of activation of other latent infections during and after courses of steroids. Both aerosol pentamidine and trimethoprim-sulfamethoxazole are effective prophylaxis. The latter appears to be more effective and costs much less, but the results of comparative trials are not yet available. More data are also needed on the safety, efficacy, and relative advantages of dapsone for prophylaxis. The first decade of the AIDS epidemic has been a decade of progress against pneumocystosis. In the next decade, the emergence of new technologies for diagnosis and of new agents for prophylaxis and treatment will bring us closer to the goal of controlling this serious infection.