A 91-year-old woman was admitted to our hospital with dyspnea. A chest radiograph and chest CT films revealed a large amount of pleural effusion in the right side pleural cavity. After serial thoracentesis, chest CT films showed a mass shadow in the right S(6). Adenocarcinoma cells were found in the pleural effusion, leading to a diagnosis of non-small cell lung cancer (stage IIIB). After administration of gefitinib, the mass shadow and pleural effusion reduced and her performance status improved. Gefitinib may be a well-tolerated therapeutic strategy in elderly and poor performance status patients with advanced non-small cell lung cancer.