Intravenous trimethoprim-sulfamethoxazole therapy was evaluated in 11 consecutive patients with documented Pneumocystis carinii pneumonia and the results compared to those from previously published studies of trimethoprim-sulfamethoxazole therapy for P. carinii pneumonia. Although six patients needed mechanical ventilation, intravenous therapy was successful in seven of 11 patients (64%), and seven of nine patients (78%) receiving 4 or more days of intravenous trimethoprim-sulfamethoxazole therapy were cured. Side effects occurred in two patients (skin rash in one, nausea and vomiting in one). A review of 80 reported cases of confirmed P. carinii pneumonia initially treated with trimethoprim-sulfamethoxazole alone revealed response rates of 67.5% in all treated patients and 85.5% in patients treated for 9 or more days. The clinical response was similar in adults (63.2%) and children (68.9%). Side effects were noted in only 11 of 80 patients (13.8%). Compared to pentamidine, trimethoprim-sulfamethoxazole has a narrower toxic-therapeutic ratio and should be preferred treatment for P. carinii pneumonia in adults as well as children.