Clinical and positron emission tomographic studies in the 'extrapyramidal syndrome' of dementia of the Alzheimer type. Extrapyramidal signs, particularly rigidity and tremor, have been reported in a proportion of patients with dementia of the Alzheimer type. To test the hypothesis that these extrapyramidal signs are similar clinically and neurochemically to the extrapyramidal signs of Parkinson's disease, a group of 20 patients satisfying clinical criteria for probable Alzheimer's disease were studied and assessed clinically for the presence of rigidity, tremor, and bradykinesia. In those patients with extrapyramidal signs, qualitative differences were observed between the signs in these patients and in subjects with Parkinson's disease. Fifteen of 20 patients underwent fluoro-18-dopa scans, which showed no significant difference in fluoro-18-dopa uptake into the caudate and putamen between normal subjects and the rigid and nonrigid patients with Alzheimer's disease, in contrast to the marked reduction in fluoro-18-dopa uptake into the putamen that is observed in Parkinson's disease. This provides clinical and in vivo neurochemical support for the hypothesis that extranigral factors may be involved in the pathogenesis of rigidity in Alzheimer's disease. 