Cases of E. coli O157:H7 infections were detected throughout Washington state in November, 1986. The outbreak was detected after a physician in eastern Washington hospitalized three patients with hemorrhagic colitis which progressed to thrombotic thrombocytopenic purpura. Illnesses occurred among patrons of a specific restaurant, and nursing homes. It was suspected that ground beef was the vehicle. Tracing of the beef led to farms where cattle were shown to be carrying E. coli O157:H7. The E. coli O157: H7 strains that the cattle carried were different from the common strain that caused the human illnesses.