A 56-year-old woman diagnosed with squamous cell lung carcinoma after a transbronchoscopic examination underwent left upper lobectomy, which revealed a pathological diagnosis of adenosquamous carcinoma containing moderately differentiated squamous cell carcinoma and bronchioloalveolar carcinoma. The epidermal growth factor receptor (EGFR) exon 19 delE746-A750 mutation was detected in deoxyribonucleic acid (DNA) isolated from specimens of both components using microdissection. Treatment with the EGFR tyrosine kinase inhibitor, gefitinib, resulted in a long-term tumor response lasting three years. Adenosquamous carcinoma is difficult to diagnose using transbronchoscopic procedures. Therefore, the examination of EGFR mutation status is important in order to determine the appropriate treatment, even in patients with non-adenocarcinoma.