Gefitinib After Chemotherapy in Treating Patients With Stage IIIB or Stage IV Non-Small Cell Lung Cancer

RATIONALE: Gefitinib may stop the growth of tumor cells by blocking the enzymes necessary for their growth. It is not yet known whether giving gefitinib after initial chemotherapy is effective in delaying the recurrence of non-small cell lung cancer. PURPOSE: This randomized phase III trial is studying gefitinib to see how well it works compared to placebo in delaying tumor recurrence in patients who have undergone initial chemotherapy for stage IIIB or stage IV non-small cell lung cancer.