A patient with signs of meningeal irritation was found to have tumor cells in the cerebrospinal fluid which were metastatic from a small primary carcinoma in the left breast. Treatment with intrathecal methotrexate in dosages up to 1 mg/kg of body weight together with systemic injections of citrovorum factor was tolerated without adverse effect. Her neurologic manifestations cleared and the cerebrospinal fluid changes reverted to normal. The patient died 5 months later of advancing systemic disease with pulmonary involvement. There was no clinical recurrence of any neurologic disorder. Since the blood‐brain barrier impedes the entrance of methotrexate into the central nervous system, the importance of administration by the intrathecal route with a large injection volume is emphasized.