 Management of leptomeningeal disease LMD in patients with metastatic breast cancer is an area of unmet clinical need. High-dose methotrexate HD-MTX is known to have activity against breast cancer and in contrast to other systemic chemotherapeutics, it penetrates the blood brain barrier, targets areas of poor cerebrospinal fluid flow, may penetrate bulky leptomeningeal disease, and provide treatment to systemic disease burden. While two retrospective studies have suggested activity of HD-MTX in LMD in patients with breast cancer, no prospective data are available to inform its inclusion in treatment regimens. Thus, while HD-MTX is included in the NCCN Guidelines for LMD and while it is used to varying degrees in cancer centers across the nation, this is more representative of the lack of available therapies for LMD as opposed to strong evidence-based data. This phase II, prospective study will evaluate systemic, intravenous HD-MTX in breast cancer patients with leptomeningeal metastasis with or without brain parenchymal metastasis. 