Six year follow up of infants with bacteriuria on screening. OBJECTIVE--To determine the value of screening for bacteriuria in infants with special emphasis on the natural course of untreated asymptomatic bacteriuria, renal growth, and renal damage. DESIGN--Prospective six year follow up of infants with bacteriuria on screening in an unselected infant population. SETTING--Paediatric outpatient clinic. PATIENTS--50 Infants (14 girls, 36 boys) with bacteriuria on screening verified by suprapubic aspiration from an unselected population of 3581 infants in a defined area of Gothenburg. INTERVENTIONS--Children with asymptomatic bacteriuria and normal findings on initial urography were untreated, although other infections were treated. MAIN OUTCOME MEASURES--Culture of urine and determination of C reactive protein concentration every six weeks for the first six months after diagnosis, every three months from six months to two years, and every six months between two and three years; thereafter yearly urine culture. Evaluation of renal concentrating capacity with a desmopressin test; radiological examination, including first and follow up urography and micturition cystourethrography without antibiotic cover; and measurement of renal parenchymal thickness and renal surface area. RESULTS--Of the original 50 infants, 37 (12 girls, 25 boys) were followed up for at least six years. Two infants developed pyelonephritis within two weeks after bacteriuria was diagnosed; the others remained free of symptoms. 45 Infants were untreated; the bacteriuria cleared spontaneously in 36 and in response to antibiotics given for infections in the respiratory tract in eight. Recurrences of bacteriuria were observed in 10 of the 50 children, of whom one had pyelonephritis. No child had more than one recurrence. At follow up urography in 36 of the 50 children (9 girls, 27 boys) after a median of 32 months no child had developed renal damage. First samples tested for renal concentrating capacity showed significantly higher values than those from a reference population (mean SD score 0.50, 95% confidence interval 0.21 to 0.79; p less than 0.001), but the last samples showed no significant difference (mean SD score 0.08, -0.24 to 0.40; p greater than 0.05). CONCLUSIONS--Mass screening for bacteriuria in infancy results primarily in detection of innocent bacteriuric episodes and is not recommended. 