Aim. The aim of this retrospective study is to investigate whether the renal cortical activity at 1st and 20th minutes in renal scintigraphy can be used as a following parameter to determine the presence of obstruction in pediatric patients group with ureteropelvic junction obstruction. Method. The consecutive 322 kidney renograms were included in the study. Standardized diuretic renograms were visually classified into three groups as non-obstructive, obstructive and indetermined according to their responses to the diuretic. In addition, the cortical regions of interest (ROI) were drawn manually at 1st and 20th min for each kidney and the cortical counts were obtained. Apart from this, kidneys were divided into three major sub-groups based on clearance half-time (T1/2); T1/2<10 min was defined as non-obstruction, T1/2>20 min as obstruction and T1/2 with a value between 10 and 20 minutes as an indeterminate result. Afterwards visual obstruction patterns and 1st/20th min. cortical activity ratios were compared. Results. When the cortical activity ratios are compared to visual obstructive pattern(nonobstructive, obstructive and indeterminate); on the Tc99m DTPA renograms the cortical activity ratios which were greater than 82.8% were obstructive, ranging from 59.3% to 82.8% were found indeterminate. On the Tc-99m MAG-3 renograms the cortical activity ratios which were greater than 74.2% were obstructive, ranging from 44.6% to74.2% were found indeterminate. As a result of the comparison of T 1/2 parameter with the obstruction patterns of the kidneys, T1/2 was in indeterminate group in 124 (41.5%) out of 299 kidneys visually evaluated as non-obstructive while in 7 (2.3%) kidneys T1/2 was in obstructive group. In 4 (23.5%) out of 17 kidneys which were visually evaluated as indeterminate, T 1/2 was in normal group, in 5 (29.4%) of them T 1/2 was in indeterminate group and in 8 (47.1%) of the 17 kidneys T1/2 was in obstructive group. Conclusion. We think that the use of 1st/20th min. cortical activity ratios on the diuretic renogram are helpful in establishing a diagnosis of obstruction in pediatric patient group.