BACKGROUND Urinary tract infection (UTI) is one of the most common bacterial infection in infants. The most severe form of UTI is acute pyelonephritis, which results in significant acute morbidity and may cause permanent renal damage. Published guidelines recommend treatment of acute pyelonephritis initially with intravenous (IV) therapy followed by oral therapy for 7-14 days though there is no consensus on the duration of either IV or oral therapy. OBJECTIVE To determine the benefits and harms of different antibiotic regimens for the treatment of acute pyelonephritis in children. METHODS We searched the Cochrane Register of Controlled Trials (Cochrane Library Issue 3, 2002), MEDLINE (1966 - September 2002), EMBASE (1988 -September 2002), reference lists of articles and abstracts from conference proceedings without language restriction. METHODS Randomised and quasi-randomised controlled trials comparing different antibiotic agents, routes, frequencies or durations of therapy in children aged 0-18 years with proven UTI and acute pyelonephritis were selected. METHODS Two reviewers independently assessed trial quality and extracted data. Statistical analyses were performed using the random effects model and the results expressed as relative risk (RR) for dichotomous outcomes or weight mean difference (WMD) for continuous data with 95% confidence intervals (CI). RESULTS Sixteen trials involving 1872 children were eligible for inclusion. No significant differences were found in persistent renal damage at six months (one trial, 306 infants: RR 1.45, 95% CI 0.69 to 3.03) or in duration of fever (WMD 0.80, 95% CI -4.41 to - 6.01) between oral cefixime therapy (14 days) and IV therapy (three days) followed by oral therapy (10 days). Similarly no significant differences in persistent renal damage (three trials, 315 children: RR 0.99, 95% CI 0.72 to 1.37) were found between IV therapy (3-4 days) followed by oral therapy and IV therapy for 7-14 days. In addition no significant differences in efficacy were found between daily and thrice daily administration of aminoglycosides (one trial, 179 children, persistent symptoms at three days: RR 1.98, 95% CI 0.37 to 10.53). CONCLUSIONS These results suggest that children with acute pyelonephritis can be treated effectively with oral cefixime or with short courses (2-4 days) of IV therapy followed by oral therapy. If IV therapy is chosen, single daily dosing with aminoglycosides is safe and effective. Trials are required to determine the optimal total duration of therapy and if other oral antibiotics can be used in the initial treatment of acute pyelonephritis.