During 2000, 1,227 cases of giardiasis were reported (24.9 per 100,000 population). This represents a 21% decrease from the 1,556 cases reported in 1999 but is similar to the mean annual number of cases reported from 1989 to 1999 (mean, 1,210 cases; range, 819 to 1,556). The median age of cases reported in 2000 was 23 years. As in previous years, cases were clustered among children less than 5 years of age (23%) and adults 30 to 39 years of age (23%); only 12% of cases were over 50 years of age. This age distribution suggests a higher risk for transmission among young children and the adults who care for them. However, cases were not interviewed systematically to identify potential sources of exposure, such as attendance at childcare facilities. One water-borne outbreak of giardiasis was identified in 2000, accounting for eight confirmed cases. The outbreak was caused by cross-contamination between a sewage system and well water.