Leptomeningeal carcinomatosis represents a rare manifestation of metastatic breast cancer (MBC). We herewith report on a patient suffering from HER2 overexpressing MBC who received intrathecal methotrexate and trastuzumab for meningeal carcinomatosis. A 48-year-old woman was diagnosed with breast cancer in December 2002. Following surgery, six cycles of adjuvant FE100C plus irradiation and, subsequently for 1 year, trastuzumab were given. As a result of disseminated metastatic spread in October 2005, the patient received whole-brain radiotherapy for symptomatic central nervous system involvement, and was put on several trastuzumab-based combination regimens (capecitabine, vinorelbine, paclitaxel). In June 2006, the patient developed clinical signs of terminal cone involvement with overflow incontinence and paraparesis of the legs. Immediate radiation led to partial relief from clinical symptoms. Subsequently, the patient was put on the tyrosine kinase inhibitor lapatinib and capecitabine (August to October 2007), but on November 6th the patient suffered again from overflow incontinence and weakness of the legs. Failing to respond to lapatinib, the patient received gemcitabine/cisplatin and, additionally, was recommenced on intravenous trastuzumab. Owing to progressive leptomeningeal disease, the patient received repeated doses of intrathecal methotrexate and trastuzumab. Within 2 weeks and four intrathecal treatments, cerebrospinal fluid cytology showed the absence of tumor cells. Moreover, a striking clinical improvement with resolution of the paraparesis of the legs and overflow incontinence was observed. This case report gives details regarding the clinical course of a breast cancer patient who received intrathecal trastuzumab and methotrexate via lumbar puncture for meningeal carcinomatosis of HER2-overexpressing MBC.