The aim of this study was to determine if it is possible to exclude renal obstruction using diuresis renography in the first 6 weeks of life (the period of physiological renal immaturity), thus avoiding unnecessary invasive procedures, such as the Whitaker test or surgery. Diuresis renography with 123I-hippuran was performed in 27 patients aged less than 6 weeks and in 50 older children who acted as a reference group (age 6 weeks to 1 year, n = 28; age 1-10 years, n = 22). All 27 patients had significant dilatation of the pelvicalyceal system on ultrasonography. Renal curves were evaluated by mathematical curve characteristics (split renal function, counts, T-max, etc.) as the visual grade of obstruction. Whole-kidney regions of interest were defined on images summed over 30 min; renal parenchyma on images summed over 5 min. The renal curves of 18/27 patients indicated tracer accumulation and led to frusemide administration. Only two patients showed no significant response to frusemide and had to be further investigated by the Whitaker test. The frequency of kidneys with no response to frusemide revealed no significant differences in the three groups. Whole-kidney evaluation resulted in an overestimation of obstruction in 9/150 kidneys, which matches the lower correlation to the DMSA separation values for this method of evaluation. In contrast with the literature, significant post-renal obstruction can be excluded by diuresis renography in most cases in spite of renal immaturity and can help to avoid invasive procedures.