To determine whether the risk factors for dementia in idiopathic Parkinson's disease (IPD) are similar to the risk factors for Alzheimer's disease, we conducted a case-control study of potential risk factors. A structured interview was administered to surrogates of 17 demented subjects with IPD and 54 nondemented subjects. Two factors emerged as possible risks for dementia. Demented patients were older than nondemented patients, although the duration of symptoms was similar. A family history of dementia was present in 30% of the demented group and 5.6% of the nondemented group. Dementia was most often reported among siblings. No difference was seen in toxic and occupational exposure, personal habits, or medical or surgical illnesses. We conclude that dementia in IPD shares some common risk factors with Alzheimer's disease. Efforts to assess the contribution of genetic susceptibility or shared environmental influences may clarify the relationship between these two diseases.