The occurrence of dementia in patients with Parkinson's disease was studied in a Parkinsonian population consisting of all traceable patients residing in a defined area. The prevalence of dementia was found to be 29 per cent in 444 patients studied. The frequency of dementia increased with advancing age and the patients showing signs of clinical arteriosclerosis were more often demented than the patients without arteriosclerosis. There was, however, an evident association between the stage of the disease and the frequency of dementia. The most severely disabled patients displayed dementia more often than the mildly affected, both among the patients with and without arteriosclerosis. The demented patients showed significantly more severe rigidity and hypokinesia when compared with the non-demented. Increasing severity of rigidity and hypolinesia, in particular was found to have a positive correlation with the degree of dementia. The association between dementia and the degree of motor involvement is considered to suggest the role of subcortical structures in the patholophysiology of dementia in Parkinson's disease.