In order to assess the familial incidence of vesicoureteral reflux (VUR) the authors examined a group of 54 families of children treated on account of this disease. Genealogical analysis revealed the same defect in two parents (1.8%), i.e. mothers and in five siblings (7.6%). Other urological abnormalities were found in 11 (6.3%) grade I relatives of children with VUR. Screening of the kidneys was made in 156 (91%) of grade I relatives. VUR was thus revealed in three siblings, incl. one where it was detected prenatally. In 18 families further abnormalities of the kidneys and urinary pathways were detected. Most frequently it was duplication of the urinary pathways which was detected in 12 grade I relatives (7%). The authors diagnosed furthermore: in mothers once right-sided hydronephrosis, once a dystopic kidney with pyelonephritic changes, once sponge kidneys, twice kidneys altered by pyelonephritis. The finding of hitherto clinically not manifested abnormalities which were thus revealed in 21 (12.1%) grade I relatives leads to the recommendation of ultrasound screening of the kidneys in families with VUR. The presence of VUR in eight (12.3%) siblings indicates the important role of genetic factors in the development of this defect.