Leptomeningeal (LM) carcinomatosis is an increasing clinical complication in patients with advanced breast cancer (BC). The LM carcinomatosis diagnostic procedures rely mainly on cerebrospinal fluid (CSF) cytology, although both the amount of CSF and the number of malignant cells remain limiting factors. Therefore, efforts should be made to design new highly sensitive diagnostic tools to detect malignant cells in CSF of BC patients with LM carcinomatosis. In this study, the human Mammaglobin (hMAM) mRNA amplification by RT-PCR was employed to detect metastatic cells in CSF and thus, to diagnose LM carcinomatosis in a BC patient. Our data demonstrate that hMAM transcripts are expressed in the CSF of a BC patient with LM carcinomatosis, hence making RT-PCR for hMAM a potentially suitable test to identify occult BC cells in the brain.