An outbreak of E. coli O157:NM infections occurred in Minnesota. The initial interviews of the case-patients revealed no common source of exposure. However, the case-control study showed that eating ground beef that had been purchased at an outlet of a major chain grocery store was associated with risk of illness. The ground beef had been purchased from eight different store outlets and varied in weight and fat content. Laboratory testing of available ground beef samples failed to show the presence of E. coli O157.