In a 3-year prospective study, 49 Italian children with the hemolytic-uremic syndrome (HUS) were examined for evidence of infection with Vero cytotoxin-producing Escherichia coli (VTEC). Diagnosis of infection was established in 37 patients (75.5%) by the combined use of stool examination for VTEC and for free fecal neutralizable Vero cytotoxin and serum analysis for antibodies to the Vero toxins and the lipopolysaccharides (LPS) of three major VTEC serogroups (O157, O26, O111). Anti-LPS antibodies were detected in sera from 30 patients: 25 had antibody to O157 LPS, 4 to O26, and 1 to O111. In as many as 27 patients (55.1%), diagnosis of infection relied only on serologic findings, and the presence of antibody to LPS was the sole evidence of VTEC infection in 20 patients (40.8%). The use of LPS from different E. coli serogroups provided evidence that in Italy O157 strains are the most prevalent VTEC involved in HUS.