Clinical and pathological data from eight cases of acute haemorrhagic leucoencephalitis (AHL) confirm the previously documented devastating features of this disease. Data from 30 cases of perivenous encephalitis (PVE) associated with viral diseases reveal pathological changes ranging from lymphocytic cuffing of vessels to severe vasculitis similar to the vasculitis of AHL. Relatively few cases show demyelination as a prominent feature. The pathological changes are unrelated to the type of underlying disease with the exception that the pathology of 'post-rubella' encephalitis tends to be mild. Two cases of rubeola and two of mumps showed viral nodules in the cortex, raising the possibility of direct viral invasion of tissue. Allowing for species differences, these changes are analogous to the pathological features of experimental allergic encephalitis (EAE); with the various pathological changes of PVE paralleling the features of ordinary EAE, while the changes of AHL and the severe cases of PVE strongly resemble hyperacute EAE.