INTRODUCTION: An aberrant or accessory crossing renal vessel (CV) leading to the lower pole of the kidney is the most common extrinsic cause of uretero pelvic obstruction(UPJ) obstruction in a child and young adolescent.There is still controversy regarding there functional significance in obstruction Preoperative identification of such vessels may influence surgical management. OBJECTIVE: First aim is to determine the value of magnetic resonance (MR) urography in detecting crossing vessels in children with UPJ obstruction,comparing the data with postoperative findings and the second one is to evaluate morphologic and functional parameters in these hydronephrotic kidneys. MATERIALS AND METHODS: Between June 2009 and December 2012 we retrospectively reviewed MR urography records of one hundred and nine children with unilateral hydronephrosis at the University Children’s Hospital. 68 (62.4%) were male and 41 (37.6%) were female,median age was 6.5 ± 5.7. Of the total number of patients, 30 (27%) underwent pyeloplasty, while 79 (72.5%) did not. The age at surgery ranged from 0.3 to 18 years (median 6.6 years). The indication for surgery was based on standard criteria (obstructed renal transit time with or without altered renal function of vDRF < 40%), and did not depend on the presence or absence of a crossing vessel.Consensus reviewes of the MR urography studies were compared with surgical findings. RESULTS: A crossing vessel (CV) was found upon surgery in 9 (33%) of 30 kidneys.On MRU, there was no crossing vessel in 21 kidneys, confirmed at surgery in 18. A crossing vessel was detected with MRU in 9 kidneys and confirmed at surgery in 6. Thus, the sensitivity of MRU was 66.7%, the specificity was 85.7%, the positive predictive value was 66.7%, the negative predictive value was 85.7% and the accuracy was 80%. There was no statistically significant difference in the detection of crossing vessels between MR urography and surgery (p = 0.004 and p < 0.01 respectively). Overall, MRU and surgery showed good agreement (κ = 0.524). CONCLUSION: Based on our results we suggest that MR urography is a reliable and safe diagnostic tool to determine crossing vessels in selected children with UPJ obstruction. Thus, MRU can substitute for other imaging modalities and provide detailed information about the morphology and function of the affected kidney.