Escherichia coli O157:H7 infection became a reportable condition in Wisconsin on April 1, 2000; previously cases were voluntarily reported by physicians and laboratories. During 1992 through 1999, 1333 cases of E. coli O157:H7 infection occurred in Wisconsin residents and were reported to the Wisconsin Division of Public Health. During this interval, the highest age-specific mean annual incidence, 13.2 cases per 100,000 population, occurred in persons 3 to 5 years old. Only 28% of patients with reported cases identified bloody diarrhea among their signs and symptoms. Of reported cases, 17% (231/1333) were involved in the eight outbreaks investigated during this interval. Among case patient identifiable risk exposures, farm related (13.4%), recreational water related (8.1%), and unpasteurized milk/dairy product (7.0%) exposures were the most frequently noted. Relatively few infections involved raw/undercooked ground beef consumption (5.8%). Recent use of pulsed-field gel electrophoresis has facilitated linkage of sporadically reported cases into recognized outbreaks. E. coli O157:H7 infections frequently occur in Wisconsin; acquisition of these infections in a wide variety of settings poses important challenges to their prevention and control.