OBJECTIVE: To calculate the frequencies of acute pyelonephritis and renal scarring in patients under the age of two, with first episode of urinary tract infection in a Brazilian university hospital, comparing with data reported in the international literature. MATERIALS AND METHODS: Scintigraphic reports of children less than two years old submitted to ^99mTc-DMSA renal scintigraphy in a university hospital in Rio Grande do Sul between 2006 and 2009 were reviewed to investigate acute pyelonephritis/renal scarring. Additionally, the presence of vesicoureteral reflux, early use of antibiotics, and comorbidities were investigated on electronic records. The sample size calculation was based on a systematic review study and obtained a minimum of 147 patients. Patients whose electronic records were not available were excluded. RESULTS: One hundred and fifty-seven children met the inclusion criteria; among them 48 had acute pyelonephritis and 8 of these had renal scars. Neither age nor sex presented any significant association with acute pyelonephritis (p = 0.405 and p = 0.124, respectively). No statistical significance was observed in the association between vesicoureteral reflux and acute pyelonephritis (p = 1.0) and other comorbidities (p = 0.470), and in relation to early use of antibiotics with acute pyelonephritis (p = 0.130) and renal scarring (p = 0.720). CONCLUSION: The frequencies found in the present study for acute pyelonephritis/renal scarring are in agreement with the results reported by most studies in the literature.

Keywords: Pyelonephritis; DMSA; Scar.