OBJECTIVES: Epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors (TKIs) are standard treatment for EGFR-mutated non-small-cell lung carcinoma (NSCLC); however, a biomarker to predict their efficacy has not been established. Although human epidermal growth factor receptor-2 (HER2) aberrations constitute a potential mechanism for acquired resistance to EGFR-TKIs, the impact of HER2 on EGFR-TKI treatment outcomes has not been systematically evaluated. In this post-hoc subgroup study, we examined the impact of HER2 on the effect of EGFR-TKIs in patients with NSCLC harboring EGFR mutations. MATERIALS AND METHODS: Of 1126 patients with NSCLC enrolled into a prospective cohort study (HER2-CS study), we analyzed data of 356 (32 %) patients with EGFR-mutant tumors. HER2 protein expression levels were determined by immunohistochemistry (IHC) with the gastric cancer criteria. Patients were divided either to an HER2-P group (HER2-IHC2+/3+) or an HER2-N group (HER2-IHC0/1+). We primarily assessed differences in the time-to-treatment failure (TTF) of EGFR-TKI between the groups. RESULTS: The HER2 scoring was as follows: IHC0 (n = 76, 21 %), IHC1+ (n = 199, 56 %), IHC2+ (n = 72, 20 %), and IHC3+ (n = 9, 3 %). The patients' demographics were similar in the HER2-P and HER2-N groups. The HER2-P group showed a significantly shorter EGFR-TKI TTF than the HER2-N group (hazard ratio [HR]: 1.657, 95 % confidence interval [CI]: 1.076-2.552; median: 13.3 vs. 19.1 months). The magnitude of the negative impact of TTF was especially dependent on performance status (PS). HER2 expression significantly deteriorated the TTF in the subgroup with PS 2 (HR: 5.497, 95 % CI: 1.510-20.02), but not in that with better PS (HR: 1.437, 95 % CI: 0.899-2.298) (pinteraction = 0.015). CONCLUSION: In the current cohort, HER2 protein expression in EGFR-mutant NSCLC may have a negative impact on the effect of EGFR-TKIs, the effect of which was PS dependent.