Environmental risk factors in siblings with Parkinson's disease. To investigate possible risk factors in Parkinson's disease, we conducted a case-controlled study of 19 families having two or more siblings with Parkinson's disease. Demographic data were collected, including lifetime histories of places of residence; sources of drinking water; occupations, such as farming; and exposure to herbicides and pesticides. Rural living and drinking well water, but not farming and herbicide exposure, were significantly increased in 38 parkinsonians compared with 38 normal control subjects. A comparison of parkinsonian siblings with siblings with essential tremor revealed no differences in any risk factors for the years of shared environment. These data suggest that living in a rural environment and drinking well water are risk factors for Parkinson's disease and that the total life exposure to an environmental toxin may be more important than exposure in early life. 