Eight laboratory confirmed E. coli O157:H7 cases in Wisconsin prompted an investigation by the Wisconsin Department of Health and Family Services. A case-control study found a strong association between illness and consumption of fresh cheese curds from Cloverleaf Farm. The farm was investigated by Wisconsin Department of Agriculture, Trade and Consumer (WDATC) Protection officials. The WDATC officials took samples of cheese, raw ingredients, packaging materials, and environmental surface swabs, as well as providing education on proper hygiene procedures. Cheese curd samples were also collected from case-patient homes, and the E. coli found in these were genetic matches to those found in stool samples of the 44 case-patients whose samples were available. It is illegal to sell cheese made with raw milk aged less than 60 days in Wisconsin, and cheese curd is always sold fresh. The dairy had apparently inadvertently used curds from raw milk cheese production and labeled them as "pasteurized." Inspection of the dairy and interviews with case-patients indicated that the dairy had not stored the cheese curds in a refrigerated area.