In September 1994, a foodborne outbreak of enterotoxigenicEscherichia coli(ETEC) infection occurred in attendees of a banquet in Milwaukee.E. coliwas isolated from stool specimens from 13 patients that were comprehensively tested; isolates from five patients were positive forE. coliproducing heat-stable toxin, were biochemically identified and serotyped asE. coliO153:H45, and were all resistant to tetracycline, ampicillin, sulfisoxazole, and streptomycin. Diarrhea (100%) and abdominal cramps (83%) were the most prevalent symptoms in 205 cases; vomiting (13%) and fever (19%) were less common. The median duration of diarrhea and abdominal cramps was 6 days and 5 days, respectively. In the United States, health care providers rarely consider ETEC as a possible cause of diarrhea in their patients, and few laboratories offer testing to identify ETEC. Hence, outbreaks of ETEC infection may be underdiagnosed and underreported. As in this outbreak, the relatively high prevalence of diarrhea and cramps lasting ≥4 days and the low prevalence of vomiting and fever can help distinguish ETEC infection from Norwalk-like virus infection and gastroenteritis due to other causes with incubation times of ≥15 hours and can provide direction for confirmatory laboratory testing.