Our aim was to investigate the clinical and pathologic characteristics of the epidermal growth factor receptor (EGFR) exon 18 mutations in East Asian lung adenocarcinomas patients. A total of 1,201 lung adenocarcinomas were analyzed for mutation in EGFR. Clinical and pathologic characteristics of patients with EGFR exon 18 mutations were compared with those who harbored classic activating mutations (exon 19 deletions and the L858R point mutation). The mutations in EGFR exon 18 were observed in 2.8% of 1,201 lung adenocarcinomas and 4.6% of patients with EGFR mutations. Patients with a single EGFR exon of 18 mutations had a worse overall survival than those harboring the complex EGFR exon of 18 mutations (p = 0.002) or those with classic activating mutations (p = 0.014). Four of five patients with EGFR exon 18 mutations showed objective response to the EGFR-TKI therapies after disease recurrence. Our results demonstrated that single EGFR exon 18 mutations may be an indicator of poor prognosis compared with complex EGFR exon 18 mutations or classic mutations. Furthermore, the results of the current study will be helpful for decision-making in the treatment of patients with EGFR exon 18 mutations.