In a mail survey, 171 Hitchcock Clinic patients with giardiasis were compared with an age- and sex-matched control group of 684 clinic patients with respect to potential risk factors. Households with shallow well or surface water sources had an odds ratio (OR) for giardiasis of 2.1 (95% confidence interval (95%CI) 1.3-3.2) compared with households with drilled well or municipal water supply. Other observed risks include family member in day care program (OR 2.2 95%CI 1.3-3.7) and family member with diagnosed giardiasis (OR 17, 95%CI 7.4-37). Previously reported risks such as travel out of country (OR 3.2, 95%CI 1.5-7.2) and camping (OR 1.7, 95%CI 0.9-3.2) were also observed. Virtually no giardiasis risk was observed associated with report of dog or barnyard animal proximity. Control for confounding and adjustment for recall and non-response bias does not materially alter the risk estimates. We suggest that shallow well or surface household water source is an important and previously unrecognized giardiasis risk factor.