Eight of 100 normotensive children who had pyelonephritic scarring secondary to urinary infection and vesicoureteric reflux were found in an earlier study to have increased plasma renin activity (PRA). Because the risk of these patients becoming hypertensive is between 10% and 20% and because renin activity may play a part in the pathogenesis of the hypertension, PRA and blood pressure were studied in 98 of the original group after five years of follow up. Two patients could not be traced, and other factors that might influence blood pressure or PRA led us to exclude 13 others, 10 of whom were girls taking oral hormonal contraceptives. Increased PRA was found in 11 of the remaining 85 patients but not in five of the eight patients with increased PRA in the first study. Of eight children identified as hypertensive in the follow up study, only three had had increased PRA five years previously. In normal children PRA decreases with age. In the initial study this tendency was less pronounced in children with renal scars, and in the follow up study it was reversed. This was also confirmed by PRA standard deviation scores, which showed a significant increase in PRA during the five years. PRA tends to rise in patients with pyelonephritic scars as they grow older. There was no direct correlation between blood pressure and PRA, plasma creatinine concentration, or degree of scarring. Analysis of blood pressure standard deviation scores, however, suggested an excessive rise in blood pressure during the five years.