DISEASE CHARACTERISTICS: Histologically confirmed stage IIIB (pleural or pericardial disease) or stage IV non-small cell lung cancer Measurable or evaluable indicator lesion that has not been irradiated Pleural effusions, bone metastases, brain metastases, elevated serum enzymes, and abnormal radionucliotide scans are unacceptable as sole indicator lesions No clinically significant pleural effusions or ascites No grade III or IV edema No prior pneumonectomy No symptomatic or uncontrolled brain or leptomeningeal involvement

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 70-100% Life expectancy: Not specified Hematopoietic: WBC at least 4,000/mm3 Hemoglobin at least 10 g/dL Platelet count at least 160,000/mm3 Hepatic: Bilirubin no greater than 1.0 mg/dL AST no greater than 1.5 times upper limit of normal (ULN) Alkaline phosphatase no greater than 5 times ULN Renal: Creatinine no greater than 1.5 mg/dL OR Creatinine clearance at least 50 mL/min Cardiovascular: No unstable cardiac disease requiring treatment Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception No other concurrent active cancer No history of significant neurologic or psychiatric disorders, including psychotic disorders, dementia, or seizures No active uncontrolled infection No other serious illness or medical condition

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy OR Progression after stable disease or initial response to 1 prior chemotherapy regimen, including 1 preoperative or adjuvant chemotherapy regimen Endocrine therapy: Concurrent steroids allowed if dose is stable Radiotherapy: See Disease Characteristics At least 3 weeks since prior radiotherapy Surgery: See Disease Characteristics Other: No prior antifolates At least 7 days since prior folic acid supplements