A case of leptomeningeal metastasis from lung adenocarcinoma is reported. In this case, we evaluated the feasibility of reverse transcriptased polymerase chain reaction (RT-PCR) methods to detect cancer cells in cerebrospinal fluids (CSF). Messenger RNA of carcinoembryonic antigen (CEA) was clearly demonstrated in CSF by reverse RT-PCR methods. An immunohistochemical study also demonstrated that tumor cells were stained positive with anti-CEA antibody. This case suggests that RT-PCR for CEA was a sensitive and useful method to diagnose leptomeningeal metastasis from lung adenocarcinoma.