A multistate outbreak of E. coli O157:H7 infections was investigated. An epidemiological study that compared foods eaten by ill persons to foods eaten by well persons showed that eating frozen, pepperoni pizza and that was produced by the General Mills Company under the brand names of Totino's or Jeno's was the likely source of the illnesses. How the pizzas became contaminated was unknown. The pizzas were produced prior to October 30 and were distributed to retail outlets nationwide. Consumers were reminded to thoroughly cook all products that are not ready-to-eat, such as frozen pizzas, according to package instructions.