Osimertinib is a highly active agent for patients with progression of lung cancer despite epidermal growth factor receptor (EGFR)tyrosine kinase inhibitor treatment. This resistance is usually due to EGFR exon 20 T790M mutation, which can be detected by repeat biopsy. We report a case in which EGFR exon 20 T790M mutation was detected by repeat ascitic fluid examination. A 71-year-old woman with lung adenocarcinoma harboring EGFR exon 19 deletion was started on erlotinib(25 mg/day)as second-line therapy. Two years later, there was increase in pleural effusion, with concomitant malignant ascites; however, pathologic examination of the pleural and ascitic fluids did not detect EGFR T790Mmutation. Afatinib(2 0mg/day) was started, but there was no decrease in the severity of ascites. Two months later, her condition was extremely deteriorated. Finally, a much larger amount of ascitic fluid obtained by paracentesis was processed for cellblock, which demonstrated EGFR exon 20 T790M mutation. Thereafter, the ascites and the primary lesion dramatically decreased after treatment with osimerti- nib(80mg/day).