An outbreak of E. coli O157:H7 occurred in both Johnson and Marion Counties, Indiana. A case-control study implicated one particular Johnson County franchise of a popular restaurant chain. Cole slaw, prepared from unwashed heads of cabbage, was identified as the vehicle responsible. E. coli O157:H7 was also recovered from a consumer sample of cole slaw and was a genetic match to the E. coli O157:H7 that had been isolated from case patients.