Seventy-two children, 59 girls and 13 boys, 0.1-15.9 (median 1.1) years of age, with acute pyelonephritis (APN) were investigated with the aid of a dimercaptosuccinic acid (DMSA) scan, renal ultrasonography (US) and a desmopressin test within 5 days of admission. Sixty-two children were reinvestigated approximately 2 months later when intravenous urography (IVU) and micturition cysto-urethrography were also performed. During infection, 92% of the children showed changes in the DMSA scan with 69% by US, and the two investigations agreed in 58% of the kidneys. At follow-up, 68% showed changes in the DMSA scan, 47% by US and 48% by IVU. The DMSA scan and IVU agreed in 60% of the kidneys. Twenty-nine percent of the children had vesico-ureteric reflux (VUR). The presence of grade greater than or equal to 3 VUR was associated with greater defects on the DMSA scan during infection, and at follow-up with a higher frequency of persistent changes compared with no VUR (P less than 0.02 and 0.01, respectively). During infection the size of the defect on the DMSA scan correlated with renal volume and C-reactive protein and inversely with the glomerular filtration rate, and at follow-up it correlated inversely with the renal concentration capacity. The DMSA scan is a sensitive method for diagnosing and localizing APN in children, and findings on DMSA scan show a weak but significant correlation with routine clinical and radiological parameters. It is suggested that persistent renal damage after APN in children without VUR may be more common than previously assumed.