To determine the maximal tolerable dosage of trimetrexate for treatment of pneumocystis pneumonia, 25 patients were treated each day with 45 mg/m2 of trimetrexate and 80 mg/m2 of leucovorin; 10 received 60 mg/m2 and 80 mg/m2; 12 received 60 mg/m2 and 160 mg/m2; and 6 received 90 mg/m2 and 160 mg/m2, respectively. Leucovorin was increased twofold and trimetrexate reduced by 50% or suspended briefly for various levels of neutropenia and thrombocytopenia until blood counts increased. Dosage-modifying hematologic toxicity occurred in 12 (46%), 8 (80%), 9 (75%), and 4 (67%) patients with the respective groups. Cytopenias were in each case reversible and other toxicities were well tolerated. All survivors but one were able to receive a full 21 doses of trimetrexate. Twenty-four (92%), 10 (100%), 7 (58%), and 4 (80%) of patients in the respective groups survived. Thus, the 45 mg/m2/day dosage of trimetrexate with 80 mg/m2/day of leucovorin resulted in the least dosage-modifying toxicity and excellent efficacy. This combination should be selected for studies to compare trimetrexate with other therapies for pneumocystis pneumonia.