We describe the case of a 69-year-old woman with advanced adenocarcinoma of the lung, in whom interstitial pneumonia was induced by gefitinib. The shadow was not clear on chest radiography, but diffuse ground-glass opacity was detected on high-resolution CT. Two treatments with pulse corticosteroids improved the lung injury temporarily. However, it became worse on reduction of the steroids. The improvement and deterioration of the pneumonia had been followed in detail by 8 examinations by high-resolution CT. The ground-glass opacity seen in the CT did not completely disappear or progress rapidly under steroid therapy. The patient died 46 days after the onset of the pneumonia. Autopsy disclosed marked cancerous invasion of the lung, and diffuse alveolar damage was also recognized in parts. The cause of death was considered to be respiratory failure due mainly to cancer progression and additionally to diffuse alveolar damage induced by gefitinib.