Regional climate change and variability and their effect on water resourceshave not been the subject of much study. Climate predictions suggest that storms will be of greater intensity and that the average precipitation event is likely to be heavier. Rainfall and runoff have been associated with individual outbreaks of waterborne disease caused by fecal-oral pathogens. Waterborne disease outbreak data from 1971 through 1994 were analyzed for groundwater and surface water in 2,105 US watersheds. Between 20 and 40 percent of outbreaks were associated with extreme precipitation. This relationship with extreme precipitation was found to be statistically significant for both surface water and groundwater, although it was more apparent with surface water outbreaks. The authors offer recommendations for improving the assessment of changes in water quality and the effect that climate variability and environmental factors have on waterborne disease risk. Includes 34 references, tables, figures.