Trimetrexate is a nonclassical antifolate with greater preclinical antitumor activity than methotrexate. Fourteen patients with stage III or IV non-small-cell lung cancer who had not previously received chemotherapy were given trimetrexate (12 mg/m2 intravenously daily for 5 days) every 3 weeks. No major objective responses were observed (95% confidence limits: 0-20%). Ten of the 14 patients had grade 2 or greater toxicity, with 50% experiencing grade 2 or greater leukopenia and/or thrombocytopenia. Nausea, vomiting, rash, mucositis, diarrhea, and serum glutamic-oxaloacetic transaminase (SGOT) elevations were also seen. At the dosage and schedule of trimetrexate used, no responses occurred in this population of patients with non-small-cell lung cancer. With the low response rate and the observed degree of myelosuppression, trimetrexate appears to have limited utility in this disease.