In November 1981, an outbreak of waterborne giardiasis occurred at a popular ski resort in Colorado. Stratification of illness by consumption of municipal tap water showed a striking dose-response, with an attack rate of 42 per cent among persons who drank six or more glasses of water per day. Filtered water samples revealed Giardia cysts in specimens both before and after treatment, and several deficiencies were found in the water treatment facility. Residents who had lived in the area greater than 2 years had a lower attack rate for illness than short-term residents.