From 23 to 26 August 1982, a gastrointestinal illness occurred among 129 to 248 (52%) persons interviewed who had attended four social events in the Minneapolis-St. Paul area. The median incubation period was 36 hours, and symptoms included diarrhea, nausea, headache, and vomiting. Findings of a food-specific questionnaire given to attendants of the four events confirmed that consumption of cake and frosting was significantly associated with development of the illness (odds ratio, 7.9 to 48.3; p = 0.006 to 0.00001). All cake items were purchased from a single bakery, where the employee who had prepared the frosting had had onset of diarrhea and vomiting on August 20. Given an approximate 60% attack rate among persons who ate frosted items, we estimate that 3000 outbreak-associated cases occurred. Serologic analysis confirmed that 17 of 25 ill persons had fourfold or greater rises in their antibody titer to Norwalk virus. Thus, foodborne transmission of Norwalk virus can result from contamination by a single foodhandler.