The case of a 42-year-old man with a cerebral glioblastoma multiforme associated with marked neovascularization of the arachnoid of the brain stem and spinal cord is reported. All of the neurological symptoms and signs were referrable to the glioblastoma and resultant craniotomies. The arachnoid contained a proliferation of well differentiated blood vessels. This neovascularization occurred in the absence of local tumor or inflammation. We suggest that the neovascularization resulted from release of a tumor angiogenesis factor into the cerebrospinal fluid.