We report a case of epidermal growth factor receptor (EGFR) inhibitor-sensitive lung adenocarcinoma with carcinomatous meningitis who showed a good response to gefitinib, an oral tyrosine kinase inhibitor of EGFR. This good response to gefitinib treatment was attributed to evidence of an EGFR mutation, L858R in exon 21, which was detected in a small amount of cerebrospinal fluid (CSF) before the positive CSF cytology. Patients with carcinomatous meningitis often have a poor performance status, and therefore diagnostic approaches and therapeutic methods are also often limited. Detection of EGFR mutations may be a useful method for non-small cell lung cancer diagnosis, and also facilitate determination of appropriate therapeutic protocols.