A case of motor neuron disease with clinical and pathological resemblance to amyotrophic lateral sclerosis (ALS) in a woman who was severely bitten on the ankle by a cat is described. Weakness first appeared at the ankles and relentlessly advanced proximally, terminating in death from pulmonary failure in a year. A number of unusual features that are uncharacteristic of ALS were found that included a markedly elevated antinuclear antibody titre in the serum and the presence of prominent oligoclonal bands in the cerebrospinal fluid. The spinal cord showed loss of anterior horn cells and pyramidal degeneration that are characteristic of ALS, but an extraordinary finding was the presence of transmural granulomatous inflammation of numerous small and medium sized vessels, especially veins, in the subarachnoid space around the cord. There were also inflammatory changes in the brainstem and spinal cord consisting of microglial and astrocytic nodules and perivenous cuffing by mononuclear cells. Ill-defined hyaline eosinophilic cytoplasmic inclusions were seen in some remaining anterior horn cells. No viral particles were found by electron microscopy despite an extensive search. Virological studies were inconclusive but there is reason to believe that this patient's illness was caused by an as yet unidentified virus.