An outbreak of E. coli O157:H7 occurred among members of an extended family (five nuclear families) who attended a summer picnic. Two persons, including the index case, from different nuclear families, had an indistinguishable strain of E. coli O157:H7 isolated from stool. Consuming sloppy joes served at the picnic was associated with an increased risk of illness. The food preparer stated that the hamburger was thoroughly cooked prior to serving. No Sloppy Joe mixture remained for testing so this food was not confirmed as the vehicle.