To assess the course of vesicoureteral reflux, we performed cystography, renal scintigraphy, and urography in all neonates with the prenatal diagnosis of renal pelvic dilation and revealed the presence of primary reflux (grades I to V) in 27 cases. Higher grades of reflux were associated with congenital renal damage, as shown by reduced tracer uptake during scintigraphy. Reflux was diagnosed more frequently in male infants (male/female ratio, 6:1), in many of whom bladder abnormalities were found by cystography. In another group of seven infants, in whom the reflux was associated with other urologic abnormalities, there was no sex prevalence. We conclude that severe primary reflux associated with hydronephrosis usually affects male infants and may be due to abnormal embryologic development of the male urethra, and that the kidney damage is primary and not the result of urinary tract infections. This pattern differs from that of vesicoureteric reflux diagnosed at an older age, which is observed most commonly in female patients.