8058 Background: Maintenance therapy after first-line platinum-based chemotherapy (first-CT) is reported to be beneficial to patients with advanced non-small cell lung cancer (NSCLC). However, its impact on overall survival appears to be marginal or negligible, if those without maintenance receive active second-line chemotherapy (second-CT), which is initiated at disease progression. The purpose of this study is to investigate the proportion of second-CT administration after first-CT for patients with advanced NSCLC. Methods: From April 2010 to September 2011, 865 patients with advanced NSCLC who were initiated on first-CT at 30 institutions in Japan were enrolled in this prospective observational study. Baseline characteristics, regimens and responses to first-CT, whether or not they received second-CT, and if not, reasons for non-administration were recorded. This report describes from patients with at least 6 months of follow up. This study was supported by the Public Health Research Center Foundation CSPOR. Results: A total of 865 eligible patients with advanced NSCLC provided patient characteristics and details of first-CT. Of all patients, 70% had adenocarcinoma, 20% had squamous cell carcinoma, and 10% were positive for the EGFR mutation. At this data cut off, 225 patients were excluded from the analysis due to disease progression and loss of follow-up during first-CT, and 194 (22%) patients received maintenance therapy after first-CT. Among the 508 patients who were followed up for at least 6 months, 131 patients (26%) could not receive second-CT; the reasons were as follows: declined PS, 79 (60%); patient refusal, 28 (21%); death of any cause, 6 (5%); others, 18 (14%). Conclusions: Preliminary results of this large observational study in Japan suggested that around 20% of patients missed an opportunity to receive appropriate second-CT despite the follow-up of advanced NSCLC patients after first-CT. Further investigation is needed to elucidate the selection criteria of patients that may benefit the most from maintenance therapy, not second-CT at disease progression.