Pneumocystis carinii is an intriguing organism found almost exclusively in the lungs. Very little is known about this organism's biologic or epidemiologic character. Over the past two decades, P. carinii has been recognized with increasing frequency as a cause of pneumonia in cancer patients, transplant recipients, and patients infected with the human immunodeficiency virus (HIV). With the increased number of cases of P. carinii pneumonia and a greater emphasis on studying this organism, sophisticated immunologic, metabolic, and molecular biologic tools have ben applied to enhance diagnosis, therapy, and prevention. Immunologic studies have identified specific antigens of human P. carinii, resulting in the development of new diagnostic tests and more specific serologic data. Metabolic studies have allowed screening and identification of new therapeutic and preventive drugs. The development of nucleic acid libraries has allowed enzymes and other proteins to be elaborated in large quantities, facilitating a wide range of studies. These new techniques have changed and will continue to change the ways that pneumocystis pneumonia is diagnosed, treated, prevented, and understood.