A 6-year-old boy developed macroscopic haematuria on the 4th day after appendectomy for acute appendicitis, at which the appendix was found to be perforated. During the next few days the urine secretion decreased and malaise, pain in the lower abdomen, nausea and vomiting occurred. On a management of ample fluid administration, the urine secretion recovered and the symptoms subsided in a few days. In the early postoperative stage after appendectomy in children the possibility should be kept in mind of the development of acute renal insufficiency due to bilateral ureteral obstruction as a result of oedema of the posterior bladder wall, even if by means of ultrasonography only mild to moderate abnormalities are noted. Awaiting decompression by means of the introduction of bilateral ureteric stents, in order to prevent irreversible renal damage, supportive therapy with fluid administration depending on the diuresis seems indicated.