Introduction Epidermal growth factor receptor (EGFR) mutations are frequently found in patients with lung adenocarcinomas, 90% being deletions in exon 19 or point mutation in exon 21. Three generations of tyrosine kinase inhibitors (TKIs) targeting EGFR mutations are available and have changed patient prognosis but less data is available on exon 18 mutations and their sensitivity to TKI therapy. Exon 18 delE709_T710insD accounts for 0.06% (16/27,294) of all EGFR mutations and is an oncogenic driver. Several partial responses to afatinib have been described. Case description We report the first case, to the best of our knowledge, of the complete response to afatinib of a 57-year-old patient with stage IV lung adenocarcinoma with a delE709_T710insD mutation in the EGFR gene detected by next-generation sequencing. Oral afatinib was prescribed and despite treatment interruptions and dosage tapering due to cutaneous adverse events, a complete response was achieved 12 months after treatment initiation and is currently maintained at 17 months. Conclusion When EGFR mutation is suspected, complete DNA sequencing of exons 18 to 21 should be carried out and we suggest that afatinib should be the first-line treatment for exon 18 delE709_T710insD-mutated advanced lung adenocarcinomas. LEARNING POINTS Rare EGFR gene mutations are not detected by standard diagnostic kits. DNA sequencing is required to diagnose rare mutations of the EGFR gene. delE709_T710insD-mutated stage IV lung adenocarcinomas respond to afatinib.