A prospective study was established within a private pediatric practice to identify the incidence and severity of vesicoureteral reflux in the siblings of patients known to have reflux. Twenty-four siblings of 18 children with reflux were studied. Eleven of these siblings were found to have reflux for an incidence of 46%. The risk of reflux in siblings of known children with reflux is thus significant and, at least in this study, compares with the risk of reflux after urinary tract infection. The outcome of the sibling group is discussed, along with our thoughts on the possible modes of inheritance of this disorder.