In vitro studies have shown that trimetrexate, a lipid-soluble analogue of methotrexate, is 1500 times more potent than trimethoprim as an inhibitor of dihydrofolate reductase from Pneumocystis carinii. Furthermore, trimetrexate is readily taken up by P carinii, while performed folates such as leucovorin are not. These observations suggest that the combination of trimetrexate plus leucovorin, which can specifically protect mammalian host tissues from the toxic effects of the antifolate, may be useful in the treatment of pneumocystis pneumonia. This concept was tested in a clinical study of 49 patients with acquired immunodeficiency syndrome (AIDS) and P carinii pneumonia who were treated for 21 days with trimetrexate and leucovorin. Patients were divided into three groups: 16 patients who were unable to tolerate or had failed both pentamidine isethionate and trimethoprim-sulfamethoxazole therapy were treated with trimetrexate plus leucovorin (Group I); 16 patients who were unable to tolerate sulfonamide therapy were treated with trimetrexate with leucovorin as initial therapy (Group II); and 17 patients in whom trimetrexate with leucovorin plus sulfadiazine was used as initial therapy (Group III). Response and survival rates were 69% and 69% in Group I; 63% and 88%, respectively, in Group II; and 71% and 76%, respectively, in Group III. Toxicity was minimal. The results indicate that trimetrexate with leucovorin is safe and effective for initial therapy in AIDS patients with P carinii pneumonia and in those intolerant or unresponsive to standard therapies.