10611 Background: Id1, a basic helix-loop-helix family member is involved in cell differentiation, proliferation, angiogenesis and metastasis in different preclinical models. Although high Id1 levels are found in poor-prognosis breast and prostate cancer human samples, whether Id1 is expressed in non-small cell lung cancer (NSCLC) and whether that potential expression has a predictive/prognostic value has never been investigated. Our objective is to analyze Id1 levels in localized and advanced NSCLC tumors and their impact on progression-free survival (PFS)/time-to- progression (TTP) and overall survival (OS). Methods: 246 NSCLC patients (pts) were studied using formalin-fixed and paraffin-embedded whole section (34) and tissue microarray (212) primary biopsies. Samples were immunohistochemically stained with the anti-Id1 monoclonal antibody 195-14 (1:500) and tumor cell expression was quantified using a validated histoscore system considering intensity and percentage of cells stained. The cut-off value to determine high vs low Id1 levels was the median histoscore. Kaplan-Meier (log rank) was used for statistical comparison. Results: 212 pts with localized NSCLC were treated with surgery (S) (187) or surgery followed by adjuvant chemo-radiotherapy (S-CT-RT) (25). 34 pts with stage IV NSCLC received a median of 4 cycles (1-8) of platinum-based chemotherapy (CT). Among the S group, higher Id1 expression levels were independently associated with a shorter OS among AC pts (p=0.017) but not other histologies. In the S-CT-RT group, the same correlation was found between Id1 and OS among AC (p=0.038). PFS showed no correlation with Id1 in either group. Consistently, in stage IV pts, Id1 maintained its prognostic value (shorter OS in pts with higher levels) only for AC (p=0.003). Moreover, Id1 also correlated with TTP after CT (lower levels, prolonged TTP) in AC pts (p=0.008). Conclusions: In our cohort of NSCLC subjects, Id1 expression was an independent prognostic factor among pts with AC but not other histologies, regardless the stage or treatment received. Moreover, the tumor levels of Id1 in pts with advanced AC may predict clinical benefit from platinum-based CT. Partially funded by Gobierno de Navarra. No significant financial relationships to disclose.