Considerable progress has been made in the identification and treatment of Pneumocystis carinii pneumonia related to acquired immunodeficiency syndrome (AIDS). Prophylaxis, either primary or secondary, represents a major advance and should be used appropriately by physicians caring for patients infected with human immunodeficiency virus. Yet with success come new challenges. Centers for treatment of AIDS that aggressively use prophylaxis have seen a shift toward severe illness due to other pathogens and generalized wasting. The strategies employed in combating P carinii pneumonia are likely to be tried against other AIDS-related infections.