We studied the clinical (10 patients) and pathological (9 patients) findings in 13 patients with herpes zoster myelitis, all of whom had systemic illnesses associated with immunosuppression. The median interval between the onset of the herpes zoster rash and myelopathic symptoms was 12 days, and the subsequent median interval to maximal deficit was 10.5 days. Presenting neurological symptoms were characteristically ipsilateral to the rash, with motor dysfunction predominating, followed by a spinothalamic and, less often, posterior column sensory deficit. Pathological involvement was most severe in the dorsal root entry zone and posterior horn of the spinal cord segment corresponding to the involved dermatome. There was variable spread both horizontally and vertically in the spinal cord. Direct varicella-zoster virus (VZV) infection of neuroectodermal cells, particularly oligodendrocytes, was demonstrated by immunostaining viral antigens (8 cases), and by the presence of Cowdry type A intranuclear inclusions (7 cases) and often was associated with focal demyelination (6 cases). In 4 patients a VZV vasculitis was associated with leptomeningitis and haemorrhagic necrosis (spinal cord in 1; brainstem or cerebellum in 3). The protracted evolution in many cases and the pathologically documented direct viral infection of the spinal cord provide a rational basis for the use of antiviral therapy in preventing or attenuating the evolving myelopathy.