Gefitinib and erlotinib are small molecules that selectively inhibit epidermal growth factor receptor (EGFR) tyrosine kinase activity. Developmental studies of either drug have failed to show synergistic effects when combined with cytotoxic drugs as the first line treatment in patients with advanced non-small cell lung cancer, but erlotinib has shown survival prolongation when compared with best supportive care in patients with recurrence. Female gender, adenocarcinoma histology and lack of smoking history are considered to be clinical factors predicting response. Being positive for EGFR mutations in exons 18-24 in cancer cells has a strong correlation with response. On the other hand, preceding idiopathic pulmonary fibrosis, male gender and history of smoking appear to be risk factors for EGFR tyrosine kinase inhibitor-induced interstitial lung disease in the Japanese population. Reports on these factors predicting response or risk for interstitial lung disease have attracted great interest in the relation between cancer genetics and drugs, as well as the relation between ethnicity and genetics. In clinical practice, EGFR tyrosine kinase inhibitor should be prescribed with careful consideration and it is essential to assess benefit and risk of the drug.