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Giardia lamblia is the most commonly reported enteric pathogen in Wisconsin. Since giardiasis became a notifiable disease, the annual number of cases reported to the Wisconsin Division of Health has increased more than 20-fold, from 2.2 cases per 100,000 population in 1981 to 49.1 cases per 100,000 population in 1988. To better understand the nature of this increasing trend, we reviewed records of G. lamblia infections reported to the Wisconsin Division of Health from 1981 to 1988. Although the increase in reported cases was a general phenomenon that was not limited to a few high-risk groups, the highest annual incidence and greatest increase occurred in children 1–4 years old; 34% of the cases in this age group occurred in children who attended day care centers. A remarkably consistent late summer (August) increase was observed across all demographic and risk groups, suggesting that G. lamblia may be more common in the environment during late summer, or that risk factors for transmission may differ during these months. Additional studies are needed to further explain the increasing incidence and seasonal nature of reported giardiasis and to identify opportunities for prevention.