Evidence is mounting that agricultural pesticide and fertilizer applications are causing groundwater contamination in some parts of the United States. A synthesis of national data has enabled researchers to identify regions potentially affected by contamination from pesticides and fertilizers and to estimate the number of people in these regions who rely on groundwater for their drinking water needs. The study found that pesticides and nitrates from fertilizers do not necessarily occur together in potentially contaminated regions. Monitoring and remedial costs, which would fall most heavily on rural people dependent on private wells, could be substantial.