- OBJECTIVES: To determine the relationship between consumption of E. coli contaminated well-water and gastrointestinal illness in rural families. METHODS: One hundred and eighty-one families with well-water as a drinking source participated in a one-year follow-up study. Water was tested for E. coli bacteria and health outcomes were monitored for house-hold members. RESULTS: E. coli in well-water was significantly associated with gastrointestinal illness in family members, however the relationship was modified by the distance from the septic tank to the well. E. coli had an odds ratio of 2.16 [95% CI 1.04, 4.42] if the septic tank was greater than 20 metres from the well and 0.46 [95% CI 0.07, 2.95] if the septic tank was within 20 metres. CONCLUSIONS: Consumption of contaminated well-water is associated with gastrointestinal illness. E. coli can be a useful marker for detecting wells that pose a potential public health problem in rural areas.