Although outbreaks of invasive disease caused by Listeria monocytogenes have been associated with ingestion of a variety of contaminated foods (1-5), most listeriosis in the United States occurs as isolated or sporadic cases. To determine the incidence of listeriosis and identify risk factors for disease, during 1988-1990, CDC collaborated with investigators in four states to conduct active laboratory-based surveillance and special studies in a population of more than 18 million U.S. residents. This report summarizes the findings of these studies (6,7).