IMPORTANCE: No studies have systematically examined the accuracy of clinical, laboratory, and imaging variables in detecting renal scarring in children and adolescents with a first urinary tract infection. OBJECTIVES: To identify independent prognostic factors for the development of renal scarring and to combine these factors in prediction models that could be useful in clinical practice. DATA SOURCES: MEDLINE and EMBASE. STUDY SELECTION: We included patients aged 0 to 18 years with a first urinary tract infection who underwent follow-up renal scanning with technetium Tc 99m succimer at least 5 months later. DATA EXTRACTION AND SYNTHESIS: We pooled individual patient data from 9 cohort studies. MAIN OUTCOMES AND MEASURES: We examined the association between predictor variables assessed at the time of the first urinary tract infection and the development of renal scarring. Renal scarring was defined by the presence of photopenia on the renal scan. We assessed the following 3 models: clinical (demographic information, fever, and etiologic organism) and ultrasonographic findings (model 1); model 1 plus serum levels of inflammatory markers (model 2); and model 2 plus voiding cystourethrogram findings (model 3). RESULTS: Of the 1280 included participants, 199 (15.5%) had renal scarring. A temperature of at least 39 degrees C, an etiologic organism other than Escherichia coli, an abnormal ultrasonographic finding, polymorphonuclear cell count of greater than 60%, C-reactive protein level of greater than 40 mg/L, and presence of vesicoureteral reflux were all associated with the development of renal scars (P = .01 for all). Although the presence of grade IV or V vesicoureteral reflux was the strongest predictor of renal scarring, this degree of reflux was present in only 4.1% of patients. The overall predictive ability of model 1 with 3 variables (temperature, ultrasonographic findings, and etiologic organism) was only 3% to 5% less than the predictive ability of models requiring a blood draw and/or a voiding cystourethrogram. Patients with a model 1 score of 2 or more (21.7% of the sample) represent a particularly high-risk group in whom the risk for renal scarring was 30.7%. At this cutoff, model 1 identified 44.9% of patients with eventual renal scarring. CONCLUSIONS AND RELEVANCE: Children and adolescents with an abnormal renal ultrasonographic finding or with a combination of high fever (>/=39 degrees C) and an etiologic organism other than E coli are at high risk for the development of renal scarring.