In an experimental study of female piglets with surgically created unilateral vesicoureteric reflux, Escherichia coli were inoculated into the bladder and, at later sacrifice, bacterial culture was undertaken of renal parenchyma from the refluxing and non-refluxing kidneys. Positive cultures of the same E. coli were obtained from 33% of refluxing kidneys with pyelonephritic renal scars, 23% of refluxing kidneys without scars and 21% of non-refluxing kidneys. Although other aspects of the experiments confirmed that a combination of urinary infection, vesicoureteric reflux and intra-renal reflux is a necessary precondition for renal scarring, these findings indicate that reflux plays a role in the pathogenesis of renal scarring over and above a means whereby pathogens gain access from the lower urinary tract to the renal substance. Possible mechanisms are discussed.