Fifty-nine human immunodeficiency virus type-1-infected patients with a microscopically proven first episode of moderate to severe Pneumocystis carinii pneumonia (PCP) were enrolled into a randomized European multicenter study. The effect of adjunctive corticosteroid (CS) therapy was assessed on (a) survival to discharge, (b) need for mechanical ventilation, and (c) survival at day 90. CS was given within 24 h of standard therapy as intravenous methylprednisolone 2 mg/kg body weight daily for 10 days. All patients received cotrimoxazole as standard treatment. Inclusion criteria were a PaO2 less than 9.0 kPa (67.5 mm Hg) and/or a PaCO2 less than 4.0 kPa (30.0 mm Hg) while breathing room air. During the acute episode of PCP, 9 (31%) of the 29 control patients died versus 3 (10%) of the 30 CS patients; p = 0.01. Mechanical ventilation was necessary in 15 patients; 12 (41%) in the control group and 3 (10%) in the CS group; p = 0.01. The 90-day survival was 69% in controls versus 87% in CS patients; p = 0.07. Based on these data we conclude that adjunctive CS therapy for moderate to severe PCP in AIDS patients reduces the acute mortality and the need for mechanical ventilation.