Background: Patients with non-small cell lung cancer (NSCLC) harboring epidermal growth factor receptor (EGFR) mutations are usually responsive to tyrosine kinase inhibitors (TKIs). However, about 20% of EGFR-mutated patients are resistant to TKIs. We analyzed the impact of TP53 mutations on response to first-line TKI treatment in EGFR-mutated NSCLC patients. Methods: 123 EGFR-mutated NSCLC patients receiving first-line treatment with a TKI were analyzed. The different TP53 mutations were evaluated in relation to disease control rate (DCR: complete response [CR] + partial response [PR] + stable disease [SD]), progression-free survival (PFS) and overall survival (OS). Results:TP53 mutations were observed in 37 (30%) patients, of which 10 (27%), 6 (16%), 9 (24%) and 12 (32%) in exon 5, 6, 7 and 8, respectively. Moreover, 24 were disruptive and 13 nondisruptive mutations. DCR was 70% in TP53-mutated patients with respect to 88% in TP53 wild type (wt) patients, (RR: 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 compared to 87% in TP53 exon 8 wt patients (RR 9.6 [2.71-36.63], p < 0.001). In patients with nondisruptive TP53 mutation, DCR was 67% with respect to 87% in patients with TP53 wt/disruptive mutations (RR 3.27 [1.14-9.12], p = 0.024). Shorter median PFS and OS were observed in patients with TP53 exon 8 mutations compared to the other patients (4.2 vs 12.5 [p = 0.058] and 16.2 vs 32.3 [p = 0.114], respectively), and these differences became significant considering the subgroup of patients with EGFR exon 19 deletion (4.2 vs 16.8 [p < 0.001] and 7.6 vs not reached [p = 0.006], respectively). COX regression analyses showed that in this subgroup of patients, exon 8 TP53mutation was associated with a higher risk of recurrence (HR 6.99 [95% CI 2.34-20.87], p < 0.001) or death (HR 4.75 [95% CI 1.38-16.29], p = 0.013) than the wild type group. Conclusions:TP53 mutations, in particular exon 8 mutations and those defined as nondisruptive, reduce responsiveness to TKI treatment and induce a worse prognosis in EGFR-mutated NSCLC patients, especially in those carrying exon 19 deletions. These results highlight the importance of tumor-suppressor genes in determining response to TKIs.