In mid-November, 1999, a cluster of children with infections caused by the same strain of E. coli O157 was investigated. Case-control studies found an association between illness and eating a beef tacos at Taco Bell restaurants. A traceback investigation implicated a beef supplier. A farm investigation was not possible because of inadequate recordkeeping by the supplier. This outbreak illustrated the importance of hospital surveillance to identify clusters and molecular surveillance to link related, geographically widespread, cases.