The nurse at a Hennepin County, Minnesota, high school notified the local health department when a large number of students reported gastrointestinal illness and missed school. Further investigation noted a significant association between attending prom on May 14 and subsequent illness. A case-control study confirmed that attending the prom was statistically associated with illness. The prom buffet was held at a Carver County country club. A variety of foods were provided. A case-control study identified two foods, chicken and pineapple, as associated with illness. The pineapple had not been washed prior to being cut and served. The chicken was held at proper temperatures. Two prom attendees had been ill at the time; one was likely to have been through the buffet line early in the service. It is likely that contamination of foods from ill students was the cause of the initial infections. Secondary infections arising from person-to-person contact likely accounted for many illnesses.