Enterohemorrhagic Escherichia coli (EHEC), have been associated with pathogenesis of hemolytic uremic syndrome (HUS) worldwide. Our aim was to determine the association of EHEC infection and HUS in Chilean children. During may 1991 and october 1993, 34 children with HUS and 33 age matched controls (children with diarrhea that did not develop HUS) were enrolled in a case/control study. For each child a stool and serum sample were obtained at admission. Stools were processed for common enteropathogens and for EHEC identification. EHEC were identified in stools by gene probes for different virulence factors (EHEC plasmid-associated fimbria, Shiga-like toxin I, Shiga-like toxin II and eae adherence factor) and by detection of free fecal toxin by a neutralization assay in Vero cells. Sera were processed for anti-cytotoxin antibodies also by an assay in Vero cells. Enteropathogens were isolated in 20.6% and 15.5% of HUS and control children respectively (p = NS). 91% of the HUS children and 73% of the control children were EHEC positive by one or more of the techniques used (p = 0.05). Of the three detection methods used for EHEC, only free fecal cytotoxin was significantly more common in HUS children than controls (45.5% vs 9% p = 0.007). Genotype patterns of HUS and controls strains were similar except for a trend towards a higher frequency of non-toxigenic strains in the control group. Serogroup O157 was more common in HUS children than in controls (9% vs 0% p = 0.036). In Chile as in other countries, EHEC infection is common and significantly associated with occurrence of HUS. Infection with EHEC strain O157 seems to be an important risk factor for HUS.