Background: Prenatally diagnosed, hydronehprosis does not mean presence of decompensated obstruction and spontaneously improvement or resolution can occur postnatally. If obstruction persists, the split renal function (SRF) of the diseased kidney will dramatically decrease, and surgical intervention will be necessary. Objective: Evaluation of outcome of pyeloplasty in children with ureteropelvic junction obstruction (UPJO) in renal unit with split function less than 20% and to asses renal function recoverability. Patients and Methods: Cohort (historical and concurrent) study included 88 patients with UPJO with split renal function (SRF) < 20%. Preoperative patient was investigated by abdominopelvic ultrasound, magnetic resonance urography (MRU) or computerized tomography and renal isotope scan. Split renal function and   T 1/2 .Preoperative and postoperative were compared. Results of the procedure were evaluated by clinical examination, abdomino-pelvic ultrasound after one month. Urine analysis with culture and sensitivity and renal isotope scan after 6 months. Results: Our study included 88 patients divided into two groups according to SRF (group 1, SRF >20% and group 2, SRF < 20%). Mean preoperative SRF was 31.34 ± 15.27%in all cases, 41.79 ± 7.03% in group 1 and 13.06 ± 4.90% in group 2, which increased postoperatively to 38.35 ± 15.61%, 46.86 ± 8.43% and 20.81 ± 7.67% respectively and this improvement was highly significant. Group 2 included 9 patient with SRF < 10%, mean preoperative SRF was 6.21 ± 2.26% which increased preoperatively to 15.04 ± 7.09% ,  also in  the remaining 23 cases, mean preoperative SRF was 15.74 ± 2.30%   increased postoperatively to  23.07 ± 6.75% . Conclusion: Poorly functioning renal unit with SRF < 20%, can show functional improvement and recoverability, so in these renal units, we should not have to rush to nephrectomy and instead of that we can proceed to pyeloplasty.