Nurses need to be aware that pneumocystosis is one of the most common and lethal opportunistic infections among AIDS patients. They are extremely susceptible because HIV impairs physiological mechanisms for microbial defense. Patients exhibit only minor symptomatology while the unchecked P carinii organisms accumulate and replicate. Eventually as the sporozoans create a physical barrier between the alveolar-capillary membranes, ventilation becomes impaired and severe hypoxemia develops. Early clinical and diagnostic studies mimic the findings characteristic of ARDS. Unless correct staining techniques are used on sputum specimens, the organism often escapes identification. Thus, the diagnosis and specific treatment of PCP is often delayed. While supporting ventilation, the treatment of choice is administration of antimetabolite drugs, either trimethoprim-sulfame-thoxazole or pentamidine isethionate. The search for more effective, as well as safer, treatment of PCP continues. Life-threatening nursing diagnoses such as impaired gas exchange urgently require priority attention. Besides physical care, the severe hypoxemia demands nursing approaches to help the critically ill patient deal with fear and powerlessness. The nurse also assumes a surrogate role to patients abandoned by family and friends. The psychosocial aspects of nursing care require enormous skill and finesse, because the blood and respiratory isolation precautions can communicate mixed messages to lonely, frightened patients.