Pneumocystis carinii is an opportunistic organism that is a common cause of pneumonia in immunocompromised patients. Its life cycle begins when cysts rupture and release sporozoites, which mature into trophozoites that eventually form cysts. The diagnostic methods for P. carinii pneumonia (PCP) have progressed from open lung biopsy to bronchoalveolar lavage (BAL) and induced sputum analysis (ISA). Detection of P. carinii organisms is done with various stains that highlight sporozoites, trophozoites, or the cyst wall. Noninvasive, cost-effective methods to aid in the diagnosis of PCP have been proposed and include chest radiograph analysis, gallium scintigraphy, serum lactate dehydrogenase levels, CD4 lymphocyte counts, pulmonary function tests, arterial blood gas analysis, and exercise hemoglobin oxygen saturation measurements. Some investigators propose empiric treatment of PCP to reduce the number of bronchoscopies performed. Most physicians prefer to make a definitive diagnosis of PCP to ensure appropriate therapy.