Traceback methods by state regulatory agencies were used to complement traditional epidemiological cluster investigation methods and confirmed hazelnuts (also referred to as filberts) as the vehicle in a multistate outbreak of Escherichia coli O157:H7 infections. Bulk in-shell hazelnut and mixed-nut purchase locations were identified during the initial epidemiological interviews. Based on purchase dates and case onset dates, regulators in Minnesota, Michigan, and Wisconsin traced product back through the supply chain. Six (86%) retail locations received the suspect hazelnut or mixed-nut shipments from a Minnesota distributor, with one retailer (14%) receiving products from a Wisconsin distributor. Both distributors received 100% of their bulk in-shell hazelnuts and mixed nuts from a distributor in California. The California distributor received 99% of their hazelnuts from a packing company in Oregon. The California distributor received the hazelnuts in 50-lb (22.7-kg) bags and either resold them without opening the bags or used the in-shell hazelnuts in the manufacture of their in-shell mixed nuts. Records at the packing company in Oregon were incomplete or lacked sufficient detail needed to identify a suspect farm or group of suspect farms. Laboratory samples collected from human cases and subsequently recalled product matched the outbreak pulsed-field gel electrophoresis subtype of E. coli O157:H7. Hazelnut harvesting practices create a plausible route of contamination from fecal matter from domestic ruminants or wild deer. This outbreak investigation demonstrates the use of product traceback data to rapidly test an epidemiological hypothesis.