A total of 47 children with acute pyelonephritis were investigated using water delay ultrasonographic equipment (Octoson) for determination of renal parenchymal volume by the stepped section technique. Thirty two patients were repeatedly investigated every to every other week up to seven weeks. Median renal parenchymal volume during acute pyelonephritis of the right kidney was 2.70 cm3/kg body weight and of the left kidney 3.10 cm3/kg; this was significantly larger than the volume of control kidneys, which was 1.82 and 2.07 cm3/kg, respectively. The most enlarged kidneys were found among the youngest children. A significant successive decrease in renal size was found during the first four to five weeks after the acute pyelonephritis. Because of enlargement of the kidneys during acute pyelonephritis we suggest that the first renal size determination to be used for following renal growth should be performed after at least four to six weeks.