MYCOSIS fungoides is a malignant proliferative disorder of reticuloendothelial cells that originates in the skin. Its presenting manifestations are variable and its course may be protracted. In the terminal stages, involvement of tissues other than the skin, such as lymph nodes, lungs, heart, liver, spleen, and the gastrointestinal tract, may be seen. Infiltration of the brain is unusual. The occurrence of such central nervous system (CNS) involvement, and the diagnostic problems presented, prompted this report. Report of a Case A 79-year-old white woman one year prior to admission, developed a dry, scaly, bluish-purple, pruritic eruption on the right wrist. It ulcerated and became painful after one month. Within a few months, new infiltrating lesions appeared over the whole body. A skin biopsy seven months prior to admission was diagnosed as mycosis fungoides. X-ray therapy (2,275 roentgens given over a 17-month period to nine sites) caused some of the