The present paper is the study of a case of progressive multifocal leucoencephalopathy that appeared in the course of a malignant reticulosis accompanied by paraproteinosis. Due to the lesions in the brain stem and cerebellum, the case evolved clinically as a pure cerebellar form. The particularity of the morphologic aspect, not as yet described, was the abundant presence of Rosenthal fibres in the older demyelination foci. These fibres exhibited a typical structure both in the electron and optic microscope. Emphasis is laid on the connection between the formation of Rosenthal fibres and blastic cellular transformation of the glial system.