Nine out of 14 patients treated with intraventricular methotrexate (MTX) for meningeal carcinomatosis from breast carcinoma and surviving more than 4 months developed disseminated necrotising leukoencephalopathy (DNL). All four patients who had received both intraventricular MTX and whole brain radiotherapy developed DNL. Five of the six patients who experienced an acute febrile reaction with mild encephalopathic signs following intraventricular administration of MTX developed DNL after a mean time of 5 months and a low mean dose of 44 mg MTX. DNL was also noted in two patients without a previous febrile reaction or whole brain radiotherapy, following prolonged intraventricular MTX therapy after a mean time of 19.5 months and a mean dose of 147 mg MTX. These findings confirm the hazards of (1) high cumulative doses of intrathecal MTX and (2) combined intrathecal chemotherapy and whole brain radiotherapy. This study also suggests a possible relationship between an early and transient febrile reaction during intraventricular administration of MTX and the development of DNL.