Background.Major urinary tract abnormalities are detected in 20 to 40% of infants with acute pyelonephritis (APN). Early detection of structural defects is essential for protecting the kidneys from reinfection and subsequent scarring. The purpose of this study was to investigate whether any factors present during the acute phase of infection could predict the presence of existing significant urinary tract abnormalities in infants.Methods.A prospective study of 180 infants, aged 1 to 24 months, with APN was conducted. Blood and urine samples were collected. Renal ultrasound (US) was performed within 0 to 6 days from admission. Final diagnosis of the urinary tract anatomy was elucidated using the results of two or more radiologic imaging studies.Results.Risk factors for the presence of significant urinary tract abnormalities in infants were pathogens other thanEscherichia coliin urine [relative risk (RR) 3.4, 95% confidence interval (CI) 2.2 to 5.3;P= 0.001], positive blood culture (RR 2.3, 95% CI 1.3 to 4.0;P= 0.039), young age (1 to 6 months) (RR 2.2, 95% CI 1.3 to 3.9;P= 0.004), lack ofpapGadhesin genes ofE. coliin urine (RR 2.1, 95% CI 1.2 to 3.9;P= 0.016) and abnormal renal US (RR 2.0, 95% CI 1.2 to 3.4;P= 0.008).Conclusions.Infants 1 to 6 months of age with APN caused by bacteria other thanE. colior bypapG-negativeE. colistrain, positive blood culture and abnormal renal US carry an increased risk for significant urinary tract abnormalities and need enforced follow-up.