Gefitinib, a selective epidermal growth factor receptor tyrosine kinase inhibitor, is an effective treatment for patients with non-small cell lung cancer (NSCLC). Some investigators have recently reported several patients complicated by acute lung injury after the initiation of gefitinib administration. In this report, we investigated the efficacy and adverse events during treatment with gefitinib. The subjects of this study were all of the 110 patients with NSCLC who were treated in our hospital and its eight branch hospitals. Patients received gefitinib at a dose of 250 mg once daily. The response rate was 30%. The frequently reported adverse events were skin disorders, gastrointestinal disturbances, liver dysfunction and acute lung injury. Five of the 12 patients who were considered to have suffered acute lung injury died of progressive respiratory failure. Of the nine patients who had pulmonary fibrosis before use of gefitinib, five developed acute lung injury during the treatment. Sera from three of the 12 patients were evaluated and all three showed increases of surfactant protein (SP)-A, SP-D and KL-6. We conclude that gefitinib was clinically useful. However, several patients suffered acute lung injury which could have been caused by gefitinib. A detection system including SP-A, SP-D and KL-6 as prime candidates as markers should be established as promptly as possible. Clinicians should be aware that treatment of NSCLC with gefitinib involves the risk of acute lung injury and therefore careful consideration should be given before deciding whether or not gefitinib is indicated for treatment. Further study is necessary to elucidate the mechanism of acute lung injury by gefitinib.