BACKGROUND: The general belief about the relation between risk of renal sequelae after pyelonephritis and age is that infants are at highest risk and children older than 5 years at lower risk. This assumption has led to differences in treatment based on age. The aim of this prospective study was to investigate the occurrence of renal lesions in children aged 0-16 years. METHODS: Between May, 1994, and January, 1996, all children aged 0-16 years who were admitted to our department with a diagnosis of probable pyelonephritis and a positive urine culture were included in this prospective study. All patients received antibiotics for 7-21 days. During the acute phase of urinary-tract infection, scintigraphy with technetium-99m-dimercaptosuccinic acid (DMSA) and ultrasonography were done. Voiding cystourethrography was undertaken at least 6 weeks after the end of antibiotic treatment. When scintigraphy showed renal parenchymal lesions, repeat scintigraphy was done after at least 2 months to assess the progression of renal lesions. For the analysis, children were grouped by age according to presumed risk of renal sequelae after pyelonephritis: high risk (< 1 year), moderate risk (1-5 years), low risk (> 5 years). FINDINGS: 201 patients were enrolled in the study (119 < 1 year, 47 aged 1-5 years, 35 > 5 years). During the acute phase of urinary-tract infection, renal lesions were found in 66 (55%) infants under 1 year, in 37 (79%) children aged 1-5 years, and in 24 (69%) children older than 5 years. Of these 127 children, 108 underwent repeat scintigraphy after an average of 3 months (50 < 1 year, 36 aged 1-5 years, 22 > 5 years). Overall, renal scars were found on repeat scintigraphy in 20 (40%) infants under 1 year, in 31 (86%) children aged 1-5 years, and in 14 (64%) children older than 5 years. 38 (36%) of these 65 patients had vesicoureteric reflux. Among 88 children who had a first documented urinary-tract infection and underwent repeat scintigraphy, renal scars were found in 20 (43%) under 1 year, in 26 (84%) aged 1-5 years, and in eight (80%) older than 5 years. INTERPRETATION: This study did not confirm the conventional view that the risk of renal scars after pyelonephritis diminishes with age. We believe that all children, irrespective of age, will benefit from any measure that prevents the development of renal sequelae.