We retrospectively studied all courses of treatment with trimethoprim-sulfamethoxazole (TMP-SMZ) alone and with adjuvant corticosteroids for AIDS-associated Pneumocystis carinii pneumonia. The corticosteroids were administered for 8-21 days (mean, 14 days) because of hypoxemia. We evaluated the influence of corticosteroids on the incidence of cutaneous adverse reactions to TMP-SMZ and on the course of AIDS during 3 months of follow-up. Of 38 patients treated with TMP-SMZ alone, 18 (47%) developed cutaneous side effects, whereas three (13%) of the 23 patients who received adjuvant corticosteroid therapy experienced such effects (P = .014). Of the 21 reactive patients, 14 were treated throughout the duration of hypersensitivity. Therapy was interrupted for seven patients (18%) treated with TMP-SMZ alone and for none of those who were given adjuvant corticosteroid therapy (P = .23). During follow-up, the incidence of mucocutaneous herpes simplex virus infection was higher among patients who received adjuvant corticosteroids than among those treated with TMP-SMZ alone (P = .005). Adjuvant corticosteroids thus reduce the incidence of adverse cutaneous reactions to TMP-SMZ in patients with AIDS who are treated for hypoxemic P. carinii pneumonia.