Hemolytic uremic syndrome (HUS) develops in 25-30% of children infected with Escherichia coli strains that produce Shiga-like toxins, also known as verocytotoxins. Mild HUS also occurs in 1 in 4 of the other family members, suggesting a familial predisposition to HUS. To understand the possible genetic predisposition, the frequency of HLA antigens was evaluated in 30 children (12 boys, 18 girls; mean age 3.8 years) with HUS following a prodrome of bloody diarrhea. When compared to a blood donor population from the same geographic area and ethnic background, no significant differences were noted in the frequency of HLA-A, HLA-C, HLA-DR, and HLA-DQ antigens. However, the frequency of HLA-B40 and its splits (B60,61 41 47) was significantly higher in the study population (corrected p < 0.005). The relative risk of developing HUS was 6.04 when HLA-B40 and HLA-B40 split products were present, and the risk increased to 8.5 when the analysis was extended to include the cross-reactive antigens B44 and B13. These HLA-B antigens share common amino acid sequences at positions 41-45 and 67-74 on the α-1 domain of the HLA class I molecule. Our data suggest that the inheritance of HLA-B40, its splits, and cross-reactive antigens increases the risk of developing HUS