Respiratory failure secondary to Pneumocystis carinii pneumonia (PCP) is associated with high mortality in AIDS patients. In a search for prognostic indicators, we reviewed hospital charts of 13 AIDS patients admitted to our intensive care service from 1982 to 1989. Mortality rate was higher from 1982 to 1987 (4/5 patients) than later (3/8 patients). There was no significant difference between survivors and non survivors of this acute episode with regard to sex. HIV risk factors, time elapsed since AIDS diagnosis, vital signs (blood pressure, heart rate, respiratory rate), hemoglobin, white blood cell count, platelets, total and T4 lymphocytes, electrolytes, lactate dehydrogenase, P24 antigen Apache II and SAPS scores and corticosteroid treatment. Higher mortality was associated with more advanced age and an oxygen alveolar-arterial gradient greater than 65 mm Hg with an inspired fraction of oxygen of 0.21. We conclude that only the alveolo-arterial gradient could represent a prognostic indicator of practical interest. It is our belief that improvement since 1988 can be explained by earlier diagnosis and prompt treatment before development of severe lung alterations.