We conducted a prospective study to determine the frequency of SLT-related HUS and SLT-related diarrhea in Argentinian children. Fecal toxin measured in a (3H) Hela cell system was detected in 10/31 (32%) HUS patients, in 7/31 (22.6%) age - season matched children with diarrhea (CD) and in 0/19 (0%) healthy children (p<0.025). Neutralization with specific antisera showed that in 5 HUS patients and in 4 CD only SLT-I was present and in 5 HUS and 3 CD SLT-I and SLT-II were found. DNA hybridization was used to screen 555 E. coli strains isolated from these patients for SLT-I and SLT-II. Three HUS and 3 CD were DNA probe positive for SLT-II and 1 HUS case was DNA probe positive for both toxins. E. coli 0157:H7 were detected in 2 CD and in 1 HUS patient. Fifty-one percent (21/41) HUS patients, 21% (10/47) CD and 5% (1/19) healthy children had serum neutralizing titers of ≥ 1:4 to shigatoxin (p<0.0005). These date show that HUS is associated with SLT-producing E. coli and suggest that the high frequency of HUS in Argentina is related to the common occurrence of SLT-associated diarrhea.