Introduction: Mutations in erb-b2 receptor tyrosine kinase 2 (HER2) oncogene are observed in approximately 3% of lung adenocarcinomas or mixed tumors with adenocarcinoma component. Activity of various biologically distinct HER2 inhibitors, including the pan-HER inhibitor afatinib, has been reported in several retrospective trials or small series in advanced pretreated NSCLC with HER2 mutations. We report the first prospective evaluation of afatinib for the treatment of this molecularly defined entity. Methods: NICHE, a single-arm phase II trial using a two-stage Simon's design, explored the potential of afatinib to control disease in pretreated patients with advanced NSCLC harboring HER2 exon 20 mutations. A total of 13 patients entered the trial and were treated with afatinib 40 mg/day until tumor progression or lack of tolerability. Results: The first-stage stopping boundary was crossed when five of nine patients did not achieve disease control at 12 weeks. The accrual into the trial was stopped with total 13 patients enrolled, with 7 (53.8%) achieving disease control at 12 weeks. Except for 1 patient with early death, progression was documented for all patients, with median progression-free survival of 15.9 weeks (95% confidence interval: 6.0–35.4), and median overall survival of 56.0 weeks (95% confidence interval: 16.3– upper limit not estimable). The toxicity profile was in the expected range. Conclusions: Afatinib did not show the expected potential for disease control in NSCLC. However, more than half of the patients in the full cohort achieved disease control at 12 weeks.