Studies of extrapyramidal motor function in patients with schizophrenia have contributed to our understanding of the phenomenology and therapeutic outcome associated with neuroleptics. An increasing body of literature suggests that extrapyramidal motor abnormalities associated with schizophrenia may be linked to the pathophysiological mechanisms responsible for schizophrenia. Similarly, it has been documented that the extrapyramidal system may be involved in motor abnormalities in patients with Alzheimer's disease (AD). The present study was undertaken to examine motor function in schizophrenia and AD patients with psychosis. Quantitative instrumental procedures were used to examine rigidity, tremor, and bradykinesia in 13 neuroleptic-naive patients with schizophrenia, 13 AD patients with psychosis, and 26 age-comparable controls. Both schizophrenia and AD patients had significantly higher tremor and rigidity scores than did normal subjects. This comparative study of schizophrenia and AD patients with psychosis suggests that the effect of dementia in patients with psychosis is to prolong movement time, whereas abnormal parkinsonian postural tremor tends to be associated with psychosis in the absence of dementia.