The clinical features and management of genuine hereditary hydronephrosis (GHH) in 4 members of the same family are presented. Genealogical studies provide evidence of a dominant autosomal inheritance with complete penetrance. All affected members of the family had inherited the same HLA haplotype through the male line. This finding, added to those from previous association studies with histocompatibility typing (in 3 families), lends support to the localization of the GHH gene/s in the 6p human chromosome. Based on our findings from the present familial study and on a review of the literature, we suggest that all first-degree relatives of children or adults with genuine hydronephrosis should be screened by ultrasound. Such a prospective screening, including fetal echography, will lead to earlier diagnosis and treatment of asymptomatic cases and, moreover, will identify GHH cases for possible genetic counseling with regard to the empiric recurrence risk.