A 58-year-old man had long-standing lesions of presumed large plaque parapsoriasis. Following treatment for nodal Hodgkin's disease (HD), these became more infiltrated, with a diagnosis of mycosis fungoides (MF). A few months later, nodules appeared on the right leg, which was lymphoedematous after inguinal irradiation for HD. Histopathological examination showed CD3+, CD30-, CD15- large pleomorphic lymphocytes, leading to the diagnosis of transformed MF. The cutaneous lesions were successfully treated with topical nitrogen mustard and interferon alfa-2b then methotrexate, but his general health worsened with depression and malaise, without specific neurological symptoms or extracutaneous spreading of the lymphoma. Cerebral computed tomographic scan revealed a cerebellar subdural collection, arachnoid cyst and quadriventricular hydrocephaly, initially considered to be non-specific. After a few weeks, clinical symptoms of intracranial hypertension appeared, and a cerebrospinal fluid (CSF) examination revealed meningeal involvement by the lymphoma. These cells were CD3-negative and the diagnosis was confirmed by polymerase chain reaction (PCR) study, which revealed an identical clonal rearrangement of the T-cell receptor gamma gene between cutaneous biopsies and the CSF. Repeated intrathecal injections of methotrexate and cranial irradiation were performed and the patient was still alive after 13 months. This case illustrates the possible meningeal involvement of MF that may be preceded by atypical and mild neurological or psychiatric symptoms, which may be dissociated from the evolution of the cutaneous lesions. Moreover, PCR study may be useful for both diagnosis and monitoring.