62 children and 51 adults with primary reflux were evaluated. Renal ratio (renal length/length of L2 plus its disc) of 99 refluxing kidneys of children and both kidneys of 51 adults were calculated. These values were compared to the mean and standard deviation of normal renal ratio of similar age groups, which were determined by measurement of 240 normal kidneys.<br>1) Cases of child: 16% of refluxing kidneys were below -2 S. D, on the initial IVPs, and most of them presented a grade 3 reflux, a grade 3 ureteral orifice and an abnormality on IVP.<br>76 refluxing kidneys were then followed for periods ranging from 1 to 8 years. Normal renal growth occurred in 68% through the priods (group 1). Renal ratio of 8 % declined temporarily below -2 S. D., although they exhibited accelerated growth and resumed normal renal ratio as soon sa spontaneous cessation or improvement of reflux occurred (group 2). All refluxing units exhibited spontaneous cessation or improvement belong to group 1 to 2, in which normal renal growth was observed.<br>Renal ratio of another 8 % were gradually deteriorated and finally became smaller than -2 S. D. (group 3). Renal ratio of 16% were persistently below -2 S. D. on initial and also on final IVP, despite the fact that half of them underwent antireflux operation (group 4). The majority of these groups 3 and 4 presented a grade 3 reflux, a grade 3 orifice and an abnormality on IVP. However, despite normal IVP or sterile urine, deteriorated renal growth were observed in some cases. 3 cases with bilateral small kidneys showed a moderate decrease in total renal function.<br>It is considered that -2 S. D. of normal renal ratio indicate the limit of conservative treatment and antireflux surgery is preferable, because a small kidney being for some period below -2 S. D. is unlikely to resume normal renal growth, while the small kidney exhibited accelerated growth when stopping or improvement of reflux occurred as soon as renal ratio declined below -2 S. D.<br>2) Cases of adult: Significant difference in renal ratio between both kidneys were commonly observed, in cases with either unilateral or bilateral reflux.<br>36% of refluxing kidneys were below -2 S. D. Two thirds of them were grade 3 reflux and one third were grade 2. But the majority presented a grade 3 orifice and an abnormality on IVP. 8 (34. 8%) cases of the 23, who had unilateral or bilateral small kidneys, were uremic. 4 cases associated with bilateral small kidneys were all uremic.<br>This study would indicate that most of small kidneys in adults are the result of gross reflux in childhood and the function of small kidney are remarkably impaired.<br>3) Four extremely samll kidneys removed (from one child and three adults) demonstrated microscopically almost normal nephron structure with slight mononuclear cells infiltration and the presence of partial chronic pyelonephritis. Slightly dysplastic elements (primitive ducts) were also observed in three kidneys. However, it could not be determined as to the reason why these kidneys were small: whether they were secondary to chronic pyelonephritis or congenital renal dysplasia.<br>Finally, it was emphasized that careful observation of renal growth is also important in management of children with reflux and they should be surgically corrected before a refluxing kidney becomes smaller than -2 S. D.