The prevalence of deposition of the Alzheimer's disease A4 amyloid protein in representative areas of the cerebral cortex was compared using a sensitive immunohistochemical technique in a group of 26 cases of idiopathic Parkinson's disease (PD) and in a control group of 82 subjects of comparable age who had died unexpectedly from non-cerebral causes. Cortical A4 protein deposition was found in 54% of PD subjects and 48% of the control group, while deposition of A4 protein in meningeal or cortical blood vessels was found in 38% of PD subjects and 25% of controls. When allowance was made for age and sex, the differences between the two groups were not found to be statistically significant. Heavy cortical A4 protein deposition was found in a number of PD cases, including two of four cases with dementia, and was absent in the other two cases. The present findings indicate that a large proportion of cases of idiopathic PD have associated Alzheimer's disease changes. In some cases these lesions are of sufficient severity to account for dementia. In other cases the changes are less severe and are probably subclinical but may be a contributory factor to the development of cognitive impairment and dementia. The absence of Alzheimer changes in some demented PD cases indicates that the dementia of PD is heterogeneous.