Leptomeningeal carcinomatosis represents a rare manifestation of metastatic breast cancer (MBC). A 39-year-old female presenting with HER2-overexpressing MBC and suffering from meningeal carcinomatosis was treated with the humanized antibody trastuzumab directed to HER2 by intrathecal administration. The patient was diagnosed with HER2-overexpressing stage III breast cancer in December 2003. In August 2004, the patient developed a singular intracerebral metastasis which was resected by neurosurgery followed by whole-brain radiotherapy. Since MRI and cerebrospinal fluid (CSF) analyses indicated meningeal carcinomatosis, the patient was commenced on trastuzumab (6 mg/kg q3w) and capecitabine (2.500 mg/m2 d1-14, q3w). Prompted by clinical deterioration, 5 repeated doses of intrathecal methotrexate (15 mg/dose) were administered, yet without clinical improvement. There is initial evidence that trastuzumab does not reach an adequate concentration in CSF after intravenous application. Nevertheless, infiltration of trastuzumab into CSF is facilitated under conditions of an impaired blood-brain barrier, as it is known for meningeal carcinomatosis. For patients with leptomeningeal disease, intrathecal application of trastuzumab may provide an interesting therapeutical approach for patients with HER2 overexpressing metastatic breast cancer. Therefore, an Ommaya reservoir for intrathecal treatment with trastuzumab was placed surgically and intrathecal therapy was begun with escalating doses of trastuzumab (5-20 mg), which proved to be effective and well tolerated by the patient. Within 2 weeks after treatment, the patients' condition improved significantly and cell counts in CSF obtained from the Ommaya reservoir remained low for 11 months after first diagnosis of meningeal carcinomatosis when clinical symptoms and MRI indicated progression of meningeal and cerebral disease.