OBJECTIVE Dynamic renal scintigraphy complemented by late gravity assisted postvoid images to 60 minutes is a frequently used diagnostic test in the evaluation of hydronephrosis. The objective of this study is to evaluate the effectiveness in acquiring images at 180 minutes to calculate the late output fraction (LOF) of 99mTc-DTPA in the diagnosis of ureteropelvic junction obstruction (UPJO). METHODS A retrospective study of 177 patients (196 renal units) of suspected cases of clinical UPJO was conducted. The patients were submitted to at least two dynamic renal scintigraphies of 99mTc-DTPA, with the addition of furosemide (F0), with a mean age of 4.3±3.8 years for the first study, and a follow-up of 2.7±2.5 years. RESULTS For diagnosis based on renal curves, a 100% sensitivity, 82.2% specificity, positive predictive value (PPV) of 10.4% and negative predictive value (NPV) of 100% were estimated. For diagnosis based on LOF, a 100% sensitivity, 96.3% specificity, PPV of 35.7% and NPV of 100% were estimated. CONCLUSIONS A LOF <10% is indicative of UPJO, and a LOF ≥15% is indicative of no UPJO. The data demonstrate that LOF presents equivalent sensitivity and NPV, and higher specificity and PPV in comparison to diagnosis based on renal curves, and is useful in the evaluation and follow-up of suspected cases of UPJO.