On three adjacent kibbutzim in the Negev (southern region) of Israel, 13 cases of Parkinson's disease were found in a population of 592 persons who were at least 40 yr of age. There were no clinical (or other) findings that distinguished these Parkinson's disease cases. Long-term residence is characteristic of this population. During the past 40 yr, water has been supplied to these persons via wells from a common aquifer. On the basis of local age-specific incidence data, no difference in age distribution was found between clustered and nonclustered cases. The incidence of Parkinson's disease is about five times greater in each of the three kibbutzim than in the remainder of the region. The three kibbutzim in the cluster use similar agricultural chemicals, as do other kibbutzim. Although associations with rural residence and well water use have been reported elsewhere, clusters of this sort have not been reported. Clusters strongly suggest that common environmental factors exist. Drinking water and agricultural chemicals are the most likely common environmental factors.