Two cases of multifocal progressive leukoencephalopathy are reported. A 62 years old patient suffering from lymphosarcomatosis for more than 5 years developed neurological symptoms and died 7 weeks after their onset. In a 44 years old patient a kidney transplantation was performed. 6 months after that operation he developed a disorder of the central nervous system and died 3 weeks later. Post-mortem examinations revealed the characteristic morphological findings of multifocal leukoencephalopathy in both the cases. Electron microscopic examination of the brain tissue obtained at autopsy demonstrated virions in paracristalline arrangement within the nuclei of oligodendrocytes. These findings are identical with those reported in literature, that were regarded as papova-viruses. Therefore, it is suggested to name the disease a papova-virus-encephalitis.