Kidney parenchymal size was estimated on urograms from 22 children with unilateral vesico-ureteral reflux (VUR), 14 children with bilateral VUR and seven children with unilateral heminephrectomy. In the bilateral VUR group, one kidney was roentgenologically normal and the other was growth-retarded. The GFR was estimated in 19 of the children. The age of the children was 3-17 years and all had a history of urinary tract infection. The size of the smaller kidney varied between 33-97% of normal. Children in the unilateral VUR group with a small kidney due to scarring and/or growth retardation showed a varying degree of compensatory hypertrophy in the contralateral kidney, which was proportional to the parenchymal reduction. This compensation was inhibited in the bilateral VUR group. There was a positive correlation between the GFR and kidney size.