Objective: To assess the evolution of individual renal function during the mat uration process in terms of single kidney glomerular filtration rate (SKGFR) and split function in children with unilateral complex renal duplication. Study des ign: We retrospectively reviewed the records of 44 children with unilateral comp lex duplex kidney. All affected kidneys had a poor or nonfunctioning dysplastic moiety, 28 in the upper pole and 16 in the lower pole. At least 2 radioisotopic examinations, including a 99mTc-mercaptoacetyltriglycine (99mTc-MAG3) renogram and a plasma clearance of 51Cr-ethylenediaminetetraacetic acid (Cr-51 EDTA), were performed in all children, the first one performed at a median age of 3 mon ths (range 2 to 15 months) and the last one at 24 months (range 12 to 120 months ). They allowed a precise estimation of split renal function, overall glomerular filtration rate (GFR), and SKGFR. Results: Mean overall kidney GFR increased si gnificantly between the two measurements from 63 ±12.7 mL/minute/1.73m2 to 95 ±21 mL/minute/1.73m 2 (P < .0001). SKGFR of the duplex side similarly increased from 26 ±7.7 mL/minute/1.73m2 to 38 ±12.6 mL/minute/1.73m2 (P < .0001). In te rms of split function, the affected kidney had a remarkable stable function betw een the two measurements, 40%±8.6 and 39%±8.3 (P = .94), respectively. Howev er, cases with the lowest initial split function ( < 30%) had the lowest initia l SKGFR and the worst further evolution. Conclusion: In children with unilateral complex renal duplication, we found on the affected side a significant increase of SKGFR because of renal maturation, whereas mean split function remained stab le during follow-up. Objective: To assess the evolution of individual renal function during the maturation process in terms of single kidney glomerular filtration rate (SKGFR) and split function in children with unilateral complex renal duplication. Study des ign: We retrospectively reviewed the records of 44 children with All affected kidneys had a poor or nonfunctioning dysplastic moiety, 28 in the upper pole and 16 in the lower pole. At least 2 radioisotopic examinations, including a 99mTc-mercaptoacetyltriglycine (99mTc-MAG3) renogram and a plasma clearance of 51Cr-ethylenediaminetetraacetic acid (Cr-51 EDTA), were performed in all children, the first one performed at a median age of 3 mon ths (range 2 to 15 months) and the last one at 24 months (range 12 to 120 months They gave a precise estimation of the split renal function, overall glomerular filtration rate (GFR), and SKGFR. Results: Mean overall kidney GFR increased si gnificantly between the two measurements from 63 ± 12.7 mL / minute / 1.73 m2 to 95 ± 21 mL / minute / 1.73 m2 (P <.0001). SKGFR of the Duplex side increased increased from 26 ± 7.7 mL / minute / 1.73 m2 to 38 ± 12.6 mL respectively, the affected kidney had a remarkable stable function betwen the two measurements, 40% ± 8.6 and 39% ± 8.3 (P = .94), respectively . Howev er, cases with the lowest initial split function (<30%) had the lowest initia l SKGFR and the worst further evolution. Conclusion: In children with unilateral complex renal duplication, we found on the affected side a significant increase of SKGFR because of renal maturation, whilst mean split function remained stab le during follow-up.