SUMMARY A case of cerebral mycosis fungoides co-existing with progressive multifocal leucoencephalopathy presented with dementia. Brain biopsy established the diagnosis of mycosis fungoides after cerebrospinal fluid examinations and computerised tomographic scanning of the brain produced non-specific abnormalities.Mycosis fungoides is a cutaneous lymphoma with characteristic histological and cytological features.' Visceral infiltration is a common necropsy finding and although the nervous system usually is spared,'-5 metastatic involvement of many parts of the nervous system and various non-metastatic neurological complications have been described (table). Cytological examination of the CSF has been helpful in
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