A 73 year old man presented with progressive choreic movement and dementia. An antineuronal antibody that recognised a 68 kDa band on a western blot was found in the patient's serum; this antibody immunolabelled neuronal somata in rat brain. Postmortem examination showed a small cell lung cancer and severe neuronal loss with lymphocytic infiltration in the striatum that was more severe in the caudate head. This is thought to be the first pathologically proved case of paraneoplastic chorea with striatal encephalitis.