A patient with mycosis fungoides developed meningeal disease while his skin disease was in remission with systemic chemotherapy. His central nervous system involvement with mycosis fungoides was controlled with intrathecal methotrexate for 7 months. The proliferating cells recovered from the spinal fluid showed similarities to the Sézary cell by light and electron microscopy. Surface receptor sutudies suggested that these cells were lymphoid cells of thymic derivation. Although mycosis fungoides has been shown to spread to the central nervous system in autopsied cases, reports of clinical neurologic disease are rare, and in only one earlier report have malignant cells have been found in the spinal fluid. Thus, as in other lymphoproliferative disorders, prompt consideration of meningeal involvement in a patient exhibiting neurologic symptoms while in peripheral remission may allow earlier treatment of this complication.