A confirmed outbreak of E. coli O157:H7 infections was associated with eating ground beef, hamburger, at McDonald's Restaurant outlets in Oregon and Michigan. Escherichia coli 0157:H7, was isolated from 9 of 12 stools collected and from a beef patty from a suspected lot of meat in Michigan. This was the first time that E. coli O157:H7 was linked to an outbreak; at that time, this serotype was thought of as rare in occurrence. The only known previous isolation of E. coli O157:H7 was from a sporadic case of hemorrhagic colitis in 1975.