A case of widespread hemorrhagic and perivenous demyelinative leukoencephalomyelitis complicating a localized herpes simplex virus (HSV) brain stem infection is reported in a 28-year-old man. The presence of the virus is documented immunohistochemically and ultrastructurally. The spinal trigeminal tract at the level of the medulla oblongata contained viral antigen in the neurons, glia and in the vascular walls, including a few endothelial cells. The foci of demyelination showed deposits of gamma globulins and slight inflammatory infiltrations; the virus was absent from these lesions. It is postulated that HSV entered the central nervous system through the trigeminal nerve. Focal expression of the viral antigen on the endothelium in a sensitized host was the likely precipitating factor in the hyperacute autoimmune reaction, resulting in the widespread hemorrhagic and demyelinating lesions in the central nervous system.