Pneumocystis carinii pneumonia (PCP) remains the most frequent life-threatening complication of HIV infection. A retrospective study was undertaken in an attempt to establish the incidence of acute respiratory failure (ARF) in AIDS-related PCP, its mortality, and the impact of adjuvant systemic corticosteroids on its outcome. Of 127 AIDS-related PCP episodes diagnosed at St. Paul's Hospital between Jan 1, 1981, and March 31, 1987, 27 developed ARF (21 percent), and the 24 who consented to ICU admission for ventilatory support were reviewed. All were given IV pentamidine or trimethoprim-sulfamethoxazole or both sequentially. Overall mortality of ARF secondary to AIDS-related PCP was 50 percent. The use of adjuvant systemic corticosteroids was associated with a decreased mortality. Of the 18 patients treated with IV hydrocortisone (400 to 1,000 mg/day in divided doses for the duration of ARF followed by a tapering regimen over 10 to 15 days), seven (39 percent) died, while five of six (84 percent) treated without corticosteroids died (p = 0.05). Survivors received ventilation for 5 +/- 2 (mean +/- SD) days and all were discharged from hospital after 20 +/- 4 days. Survivors were also younger (34 +/- 8 vs 43 +/- 10 years, p = 0.034) and presented earlier (14 +/- 3 vs 34 +/- 7 days after onset of symptoms p = 0.017). Known AIDS, previous PCP episodes, and arterial blood gas values at the onset of ARF did not correlate with outcome. We conclude that ARF secondary to AIDS-related PCP merits aggressive management. In particular, younger patients presenting early after the onset of respiratory symptoms appear to have a better prognosis. The decreased mortality associated with the use of adjunctive corticosteroids supports the need for prospective controlled evaluation of this therapeutic modality.