Mycosis cells were identified in the pre-morbid cerebrospinal fluid of a patient with neurological symptoms and mycosis fungoides (MF). Light and electron microscopic examination at autopsy confirmed leptomeningeal involvement by mycosis fungoides. The cellular morphology of the non-cutaneous infiltrates supports the concept that mycosis fungoides retains a unique histopathology in its dissemination to the viscera. The importance of cerebrospinal fluid cytology in patients with mycosis fungoides is emphasized.