We report the case of a Japanese male with epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI)-sensitive lung adenocarcinoma, who had an EGFR mutation and presented in the emergency department with acute cardiac tamponade as the recurrence during EGFR-TKI therapy. We could detect a second mutation, T790M in exon 20 in the pericardial effusion. This is the first report to detect the resistant mutation T790M in pericardial effusion. We suggest that the pericardial effusion may therefore be useful as surrogate tissue for detecting EGFR mutation.