IN the course of the regular post-mortem examination of the brains of patients coming to autopsy at the Massachusetts General Hospital, my attention has been called in recent years to an unusual disorder of the cerebral white matter with distinctive features unfamiliar to me and my colleagues from our own experience or that of others. We reported 3 such cases in 1958¹ and called the condition "progressive multifocal leukoencephalopathy" to emphasize the outstanding clinicopathological features. Since these 3 cases, and 3 of the 5 others previously reported, were associated either with lymphatic leukemia or with Hodgkin's disease, we considered the . . .