The occurrence of the hemolytic uremic syndrome in three siblings prompted us to study this illness in families. Two groups of patients could be identified when 83 siblings with the syndrome in 41 families were examined. Siblings whose onset occurred within a short time of each other had a relatively good prognosis (19 per cent mortality); those whose onset was more than a year apart had a poorer prognosis (68 per cent mortality). We speculate that an environmental agent may have caused the syndrome in the first group, and that genetic factors may have been important in the second.