Objective. To identify dietary risk factors for sporadic listeriosis. Design. Case-control study with blinded telephone interviews. Setting. Multistate population of 18 million persons, November 1988 through December 1990. Participants. One hundred sixty-five patients with culture-confirmed listeriosis and 376 control subjects matched for age, health care provider, and immunosuppressive condition. Results. The annual incidence of invasive listeriosis was 7.4 cases per million population; 23% of the infections were fatal. Cases were more likely than matched controls to have eaten soft cheeses (odds ratio OR, 2.6; 95% confidence interval CI, 1.4 to 4.8; P = 0.002) or food purchased from store delicatessen counters (OR, 1.6; 95% CI, 1.0 to 2.5; P = 0.04); 32% of sporadic disease could be attributed to eating these foods. Sixty-nine percent of cases in men and nonpregnant women occurred in cancer patients, persons with the acquired immunodeficiency syndrome, organ transplant recipients, or those receiving corticosteroid therapy. Among these immunosuppressed patients, eating undercooked chicken also increased the risk of listeriosis (OR, 3.3; 95% CI, 1.2 to 9.2; P = 0.02). Conclusions. Foodborne transmission may account for a substantial portion of sporadic listeriosis. Prevention efforts should include dietary counseling of high-risk patients and continued monitoring of food production