IRESSA was dogged by misfortune due to it's serious side effect, interstitial pneumonia, specially more common in Japanese patients with lung cancer. Afterward, it developed if the patient is properly selected according to his clinical backgrounds, such as Asian, non-smoker, female and adenocarcinoma. IRESSA is effective in 80% of patients and relatively safe from serious side effects. Further, the drug is expected to be safe and effective if the patient is selected according to the genetic status of cancer cells. Prof. Yoichi Nakanishi is jointing to some clinical studies on IRESSA such as IRESSA Pan-Asian Study, LOGIK (66 institutions in Kyushu, Japan) study, and Korean-Japanese intergroup study (KASLC and LOGIK). In these studies, gene mutaion analysis is adopted to select a proper patient to use IRESSA. In near future, genetic analysis would become a powerful tool to precisely predict the effect and the safety of drugs.