Pemetrexed Disodium Plus Gemcitabine in Treating Patients With Locally Advanced or Metastatic Non-Small Cell Lung Cancer

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed locally advanced or metastatic non-small cell lung cancer * Stage IIIB disease that is ineligible for combined modality therapy OR * Stage IV disease * Measurable disease * No clinically detectable (by physical exam) third-space fluid collection (e.g., ascites or pleural effusions) that cannot be controlled by drainage or other procedures * No brain metastases (even if treated) PATIENT CHARACTERISTICS: Age: * 18 and over Performance status: * ECOG 0-1 Life expectancy: * At least 12 weeks Hematopoietic: * Absolute neutrophil count at least 1,500/mm\^3 * Platelet count at least 100,000/mm\^3 * Hemoglobin at least 9 g/dL Hepatic: * Bilirubin no greater than 1.5 times upper limit of normal (ULN) * AST and ALT no greater than 3 times ULN (5 times ULN if liver metastases present) * Alkaline phosphatase no greater than 3 times ULN (5 times ULN if liver metastases present) Renal: * Creatinine clearance at least 45 mL/min Other: * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception * Able to take folic acid or cyanocobalamin (vitamin B12) supplementation * No uncontrolled infection * No concurrent chronic debilitating disease * No weight loss of 10% or more within the past 6 weeks * No other prior malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or noninvasive carcinoma PRIOR CONCURRENT THERAPY: Biologic therapy: * No prior biologic or genetic therapy for lung cancer * No concurrent immunomodulating agents Chemotherapy: * No prior chemotherapy for lung cancer Endocrine therapy: * Not specified Radiotherapy: * At least 4 weeks since prior radiotherapy to a different site and recovered * No prior radiotherapy to 25% or more of bone marrow * No prior radiotherapy to whole pelvis * No prior radiotherapy for primary disease * No concurrent radiotherapy Surgery: * More than 4 weeks since prior major surgery Other: * No aspirin or nonsteroidal anti-inflammatory agents 2 days before, during, and for 2 days after pemetrexed disodium administration (5 days for long-acting agents such as piroxicam, naproxen, diflunisal, or nabumetone) * No other concurrent cytostatic or cytotoxic therapy