Temporary extrapulmonary oxygenation may benefit selected patients with Pneumocystis carinii pneumonia who exhibit severe respiratory insufficiency. Four persons were considered candidates for extrapulmonary oxygenation with a membrane lung while under treatment for pneumocystis pneumonia. In one patient, attempts to institute membrane lung circulation were postponed until his condition was terminal. In another individual, membrane lung support was discontinued prematurely because of complications of anticoagulation. A third patient died of cardiac failure even though her oxygenation had improved during respiratory assistance. In the fourth, the membrane lung was used successfully to maintain the patient through therapy until lung recovery was adequate to resume vital function. The four cases presented are examples of the immunosuppression that creates susceptibility to pneumocystis pneumonia: In two patients, immunodeficiency was caused by lymphoma and combination chemotherapy for the underlying disease; in two others, immunosuppression was induced for the purposes of transplantation. Two patients underwent veno-venous perfusion for prepulmonary oxygenation, and one underwent venoarterial bypass with the membrane lung. Indications for, and techniques of, membrane lung bypass are reviewed. This method of extrapulmonary membrane lung support may save some patients with transient severe pulmonary insufficiency due to P, carinii pneumonia, and the membrane lung may be an adjunct to antimicrobial therapy.