10-ethyl-10-deaza-aminopterin (10-EDAM) is an analog of methotrexate that differs from its compound by modification of the N10 position and demonstrates greater preclinical antitumor activity and less toxicity. In this phase II trial, 20 patients with stage III or IV non-small-cell lung cancer (NSCLC) were administered 10-EDAM at a dose of 80 mg/m2 once weekly for 5 weeks. No patient had previously received chemotherapy. Nineteen of the 20 patients were adequately treated for response assessment. Six of 19 patients (32%) experienced a major objective response (exact 95% confidence limits, 15% to 55%). The median duration of response has not been reached and will exceed 13 months. Mucositis was the most common toxicity observed. Leukopenia was seen in only 10%, and 15% had platelet nadirs less than 100,000/microL. At the dosage and schedule used, 10-EDAM is an active agent in patients with NSCLC who are previously untreated with chemotherapy with a predicted response rate greater than or equal to 15% (P = .05). Because of its level of antitumor activity and the fact that 10-EDAM causes minimal myelosuppression, it is a suitable agent for further study in combination with other chemotherapeutic agents in this disease.