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Purpose: To analyze the impact of TP53 mutations on response to first-line tyrosine kinase inhibitors (TKI) in patients with EGFR-mutated non–small cell lung cancer (NSCLC). Experimental Design: 136 EGFR-mutated NSCLC patients receiving first-line TKIs were analyzed. TP53 mutations were evaluated in 123 patients in relation to disease control rate (DCR), objective response rate (ORR), progression-free survival (PFS), and overall survival (OS). Results: TP53 mutations were observed in 37 (30.1%), 10 (27.0%), 6 (16.2%), 9 (24.3%), and 12 (32.4%) patients in exons 5, 6, 7, and 8, respectively. DCR was 70% in TP53-mutated patients compared with 88% in TP53-wild type (wt) patients [relative risk, RR, of disease progression: 3.17 (95% CI, 1.21–8.48), P = 0.019]. In particular, a 42% DCR was observed in patients with TP53 exon 8 mutation versus 87% in exon 8 wt patients [RR of disease progression 9.6 (2.71–36.63), P < 0.001]. Shorter median PFS and OS were observed in patients with TP53 exon 8 mutations compared with others (4.2 vs. 12.5, P = 0.058, and 16.2 vs. 32.3, P = 0.114, respectively); these differences became significant in the subgroup with EGFR exon 19 deletion (4.2 vs. 16.8, P < 0.001, and 7.6 vs. not reached, P = 0.006, respectively), HR 6.99 (95% CI, 2.34–20.87, P < 0.001) and HR 4.75 (95% CI, 1.38–16.29, P = 0.013), respectively. Conclusions: TP53 mutations, especially exon 8 mutations, reduce responsiveness to TKIs and worsen prognosis in EGFR-mutated NSCLC patients, mainly those carrying exon 19 deletions. Clin Cancer Res; 23(9); 2195–202. ©2016 AACR.