Lung adenocarcinomas often metastasize to the brain, and the prognosis of patients with brain metastases is still very poor. The epidermal growth factor receptor (EGFR) gene is mutated in a considerable fraction of primary lung adenocarcinomas, in particular those with drastic response to EGFR tyrosine kinase inhibitors. The present study was designed to elucidate the prevalence of EGFR mutations in brain metastases and the timing of their occurrence during cancer progression. EGFR mutations were detected in 12 of 19 metastatic lung adenocarcinomas to the brain (63%). This frequency was higher than those in previous studies for EGFR mutations at various stages of lung adenocarcinoma in East Asia, including Japan (i.e., 20-55%). In 6 cases with EGFR mutations, the corresponding primary lung tumors were also examined for the mutations, and in all of them, the same types of EGFR mutations were detected also in the primary tumors. In 2 of them, second metastatic brain tumors in addition to the first ones were also available for analysis, and the same types of EGFR mutations were detected in both the first and second ones in both cases. These results indicate that EGFR mutations are present frequently in brain metastases and occur preceding brain metastasis. These findings will be highly informative for treatment of metastatic lung adenocarcinoma to the brain.