Severe Pneumocystis carinii pneumonia (PCP) necessitating mechanical ventilation has been associated with a high mortality rate in several previous studies. This has discouraged physicians from recommending, as well as patients from accepting, mechanical ventilation when respiratory failure developed. Analysis of 19 records of patients with first episode PCP who were intubated and received mechanical ventilation showed a mortality of 57.8%. A constellation of clinical and laboratory findings was found that identified those patients more likely to survive, including a shorter duration of symptoms prior to admission, better arterial oxygenation on admission, deterioration of respiratory status soon after bronchoscopy, decrease in serum lactic acid dehydrogenase and rapid improvement in arterial blood gas determinations after institution of mechanical ventilation. Mean survival for the group that recovered from PCP after mechanical ventilation was not different from that of other patients with PCP who did not require mechanical ventilation. Thus, patients at risk for PCP should be advised to present to medical facilities as soon as symptoms of this disease develop and, when appropriate, should be encouraged to undergo intubation with respiratory support if it becomes necessary.