During 2001, 1,061 cases of Giardia lamblia infection (21.5 per 100,000 population) were reported. This represents a 14% decrease from the 1,227 cases reported in 2000 but is similar to the mean annual number of cases reported from 1990-2000 (mean, 1,192 cases; range, 827-1,556). The median age of case-patients reported in 2001 was 22 years. Twenty-one percent of cases were in children less than 5 years of age. Only 10% of case-patients 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 systematically interviewed to identify potential sources of exposure, such as attendance at child daycare facilities or caring for young children. No food-borne or water-borne outbreaks of giardiasis were reported in 2001.