We studied the molecular epidemiology of the recent fast-food restaurant chain-associated Escherichia coli O157:H7 outbreak in Washington State. Genomic DNAs prepared from strains isolated from 433 patients were probed with radiolabelled Shiga-like toxin (SLT) I and SLT II genes and bacteriophage l DNA and were subsequently analyzed for their restriction fragment length polymorphism (RFLP) patterns. The SLT RFLP and l RFLP profiles of an E. coli O157:H7 strain isolated from the incriminated beef and prototype patient were compared with those of the patient isolates for determination of the concordance between patterns. Of the 377 patients with primary and secondary cases of infection epidemiologically linked to the outbreak, isolates from 367 (97.3%) of the patients displayed SLT RFLP and l RFLP profiles identical to those of the outbreak strains. Isolates from 10 of the 377 (2.6%) patients possessed SLT RFLP and l RFLP profiles different from those of the outbreak strains, and the patients from whom those isolates were obtained were subsequently characterized as having non-outbreak-related infections. The E. coli O157:H7 strains isolated from 31 of 44 (70.4%) patients who were epidemiologically excluded from the outbreak were linked to the outbreak by RFLP typing. Our results indicate that SLT RFLP and l RFLP analyses are stable and sensitive methods, and when they are used in conjunction with an epidemiological investigation they could result in an earlier recognition of outbreaks and their sources, hence prompting measures to prevent the continued transmission of E. coli O157:H7.