Background: Molecular profiling is often limited by access to sufficient tumour tissue for comprehensive analysis and due to tumour heterogeneity, the complete range of tumor DNA abnormalities may not be represented nor accurately reflect the clinical evolution of disease. Circulating tumour DNA (ctDNA) can be used as a liquid biopsy for molecular abnormalities detection, quantification and monitoring for personalised treatment strategies. Methods: Plasma was collected at baseline (BL) and during study therapy from advanced NSCLC patients (pts) enrolled in a placebo controlled phase III trial of a novel irrerversible EGFR inhibitor; all patients had received standard therapy with chemotherapy and gefitinib or erlotinib. Archival tissue was collected when available but biopsy was not required prior to enrolment. BL Plasma ( < 3ml), stored for ~8 years was used to extract DNA and analysed using InVision (enhanced tagged-amplicon sequencing). Results: BL plasma from 387 pts was tested; 289 pts had available tissue results (from archival tissue collected at diagnosis) for EGFR (174WT/115Mut) and 243 for KRAS (205WT/38Mut). Despite age of plasma samples, ctDNA analysis detected cancer mutations in 310 pts (82%): TP53 (45%),KRAS (15%), and EGFR (43%; 56% were EGFR del19 and 29% L858R); T790M was detected in 80 patients. EGFR mutations were identified in 29 patients and KRAS in 10 patients with unknown tissue status. Also of note, STK11 (32 pts, 12 with KRAS), BRAF (5pts, 3 with V600E), MET(7 pts, 4 with MET_amp), ERBB2 (16pts, 10 with ERBB2_amp) were identified in ctDNA analysis. Median time and median number of lines of systemic therapy between tissue biopsy and blood was 714 days and 3 lines respectively. Further analyses of ctDNA analyses in context of patient and trial outcomes are in progress. Conclusions: Liquid biopsies provide opportunity to evaluate molecular mutation profile in NSCLC patients. We demonstrate a highly sensitive method for ctDNA analysis which is complementary to tissue molecular analysis.