A screening survey for asymptomatic bacteriuria (ASB) in 13464 schoolgirls aged 4 to 18 years in Newcastle upon Tyne showed an overall prevalence of 1.9%. In girls 4 to 6 years it was 1.4%, in girls aged 7 to 11 years it was 2.5%, in girls aged 12 to 18 years it was 1.6%, a statistically significant rise and fall. Renal scarring was found in 39 (15%) of 254 girls with ASB. Neither the prevalence nor the severity of renal scarring increased with age. There was no association between ASB and social class. Of the 254 girls with ASB, 24% had no symptoms. Infections with klebsiella were more frequently associated with renal scarring than infections with Esch. coli. The prevalence of ASB in 1595 boys aged 5 to 18 was low, 0.2%. Ureteric reflux was present in 15% of girls without renal scarring and in 46% of those with renal scarring (P less than 0.001). Renal scarring also showed a significant association with duplication, hydroureter, or single saccules in the bladder. The greater the severity of scarring the more frequently was reflux present. This study gave no evidence that asymptomatic bacteriuria leads to progressive renal damage during childhood. It therefore does not support the recommendation for prescriptive screening of schoolchildren, but emphasizes the need for prospective studies of the natural history of ASB.