Univariate and multivariate analyses were applied to determine risk factors for the progression of Escherichia coli O157:H7 enteritis to hemolytic uremic syndrome (HUS). Both clinical and laboratory variables were assessed for 118 pediatric patients (28 HUS; 90 enteritis only). Verotoxins 1 and 2 were produced by 89% of E. coli strains whereas verotoxin 2 only was produced by 11%. Although a greater frequency of strains producing verotoxin 2 only occurred in HUS isolates (p = 0.11), toxin phenotype was not significantly associated with risk after multivariate analyses. HUS patients with or without neurological manifestations had similar frequencies of the two toxin phenotypes among their isolates. Significant associations for young age (RR = 0.984; 95% CI = 0.971-0.998) and prolonged use of antidiarrheal agents (RR = 44.11; 95% CI = 8.48-229.4) with HUS were apparent. A lesser chance of progression was observed for patients whose strains possessed a 4 kb plasmid (RR = 0.27; 95% CI = 0.08-0.94). Our results are consistent with the hypothesis that progression to HUS is dependent upon both bacterial virulence factors and the clinical characteristics of the individual patient.