Ninety waterborne outbreaks and 23,776 cases of giardiasis have been reported in the United States through 1984. Approximately 69% of the outbreaks and 74% of the cases occurred in community water systems. Most outbreaks (57%) and cases (52%) resulted from the use of contaminated surface water which had not been treated or had been treated only by simple chlorination. Fifteen outbreaks (17%) and 7,440 cases (31%) were associated with operational problems on inadequate design of water filtration plants. A small but significant (15%) number of cases of giardiasis resulted from outbreaks caused by contamination of water mains through cross-connections, damage of mains, and repair of mains.