Two families with all children affected by primary vesicoureteral reflux are reported. Both sets of parents were examined and only in one family the mother was also affected by unilateral reflux. This congenital lesion has only recently been recognized as a common disease, but because diagnosis depends on voiding cystography, it is not made until the child or adult becomes symptomatic or presents with end-stage renal disease. As most cases are sporadic, a multiple factorial mode of inheritance is most probable, but an autosomal dominant or recessive gene cannot be excluded as the cause of the disease in some families. It is recommended that all first degree relatives of the patients should be investigated, in order to detect asymptomatic cases of this condition, which is likely to cause progressive renal damage and may have fatal termination due to renal failure.