Involvement of the brain, spinal cord, peripheral nerves, meninges, and cerebrospinal pathways in mycosis fungoides manifests itself, clinically, as an obscure neurologic disturbance, occurring in patients with cutaneous manifestations of that disorder. Neurologic expressions of this particular cutaneous lymphoma are infrequently reported, in spite of the fact that many patients dying in the course of mycosis fungoides have pathologically demonstrable central nervous system, peripheral nervous system, and meningeal involvement. We report here a patient who incurred devastating neurologic illness as one of the initial features of the course of systemic dissemination of mycosis fungoides. Death occurred nine years after the onset of cutaneous illness. In this instance, cerebrospinal fluid cytologic studies established the diagnosis of nervous system involvement. Mycosis fungoides cells shed into the cerebrospinal fluid were studied by special techniques of light and electron microscopy. Brain scan abnormalities were present, which were clinically correlative with foci of brain involvement. The initial response of patient's nervous system illness to therapy was gratifying but unsustained, and she went on to fatal brain and multiple organ involvement with disseminated mycosis fungoides. The techniques of spinal fluid cytologic diagnosis, confirmatory of the etiology of disease in this case, may have practical appications in the diagnosis of otherwise obscure neurologic disorders.