Grade 5 vesicoureteral reflux (VUR) is almost exclusively seen in male infants, and in one-third of cases occurring with a generalized small kidney with decreased renal function without a previous history of urinary tract infection. In females, however, high-grade reflux is rare and kidney damage almost always less severe and of the focal type, as in older children. Assessment of the bladder function with urodynamic and free voiding studies also indicates a difference between male and female reflux during infancy. Half of the males with dilating reflux initially have a hypercontractile urodynamic pattern indicating small functional capacity with high voiding pressures and often instability during filling. This pattern changes during the first couple of years to high-capacity overdistended bladders, often with incomplete emptying. In females, hypercontractility is seldom seen, but bladder function is characterized by high capacity and there is an increase in residual urine from presentation. The pathogenesis of VUR has also been suggested to differ between the sexes. Transient anatomical obstruction during fetal life has been proposed as the cause of gross VUR in males. The spontaneous resolution rate of dilating infant VUR seems to be significantly higher than in older children. A resolution of 40% of grades 4 and 5 has been reported during the first couple of years in prenatally diagnosed cases, suggesting that antireflux surgery should be postponed until after the infant year.