Many AIDS patients develop PCP, but the effectiveness of early diagnosis and treatment of PCP is not known. The effectiveness of PEN may be improved if treatment is begun when the parasite burden (the number of organisms in the lung) is still small, and before respiratory symptoms appear. If PFT of HIV-infected patients is able to identify patients in the early stages of infection, outpatient treatment of these patients offers a possible alternative to the expense and toxicity of continuous preventive therapy of all high-risk patients.