Objective To evaluate the efficacy of flexible ureteroscopyfor the treatment of the upper urinary tract calculi in children.Methods There were 21 children with 12 males and 9 females included in this study.The mean age was 7.2 (range 2.3 to 16.0) years.Among them,12 had upper ureteral calculi (10 with calculi in situ,2 with middle/distal ureteral calculi shifting to upper ureteral after rigid ureteroscopic lithotripsy) and 9 had renal calculi.Ipsilateral mild to moderate hydronephrosis was found in all of the cases.Four children had melamine-induced stones (3 with upper ureteral calculi and 1 with renal calculi).The calculi were found on left side in 11 cases,on right side in 7,and on both sides in 3 (upper ureteral calculi).The mean stone size was i.0 (range 0.6 to 1.5) cm.Retrograde flexible ureteroscopy and anterograde flexible ureteroscopy in mini-percutaneous nephrolithotomy was performed.Results Eighteen cases were performed retrograde flexible ureteroscopic procedure.One case was conversed to mini-percutancous nephrolithotomy because the flexible ureteroscope could not be inserted into the upper ureter.The flexible ureteral access sheath was failed to insert into the upper ureter in 2 cases,then the flexible ureteroscope was inserted into ureter directly.The stones were successfully fragmented after two stages in these two cases.The successful rate of stone search was 100％ in 17 cases.Fifteen cases were successfully performed in one stage.The success rate of stone fragmentation was 88％ with the holmium laser lithotripsy.Three anterograde flexible ureteroscopy with mini-percutaneous nephrolithotomywere successfully performed,and the stones were successfully found and fragmentated after a single holmium laser lithotripsy.The mean operative time was 35 (range 20 to 70) min.There was no major perioperative complication.The patients were discharged from hospital after a mean of 4.5 days (range 3 to 7).Double-J stent was removed after 2 to 4 weeks when no residual stones more than 2.0 mm was found.No recurred stones,no urethra stricture,no ureter stricture,no urinary incontinence and no vesicoureteral reflux was found during the 3 to 72-months follow-up.Conclusions Flexible ureteroscopy could be a safe and feasible method for the treatment of the upper urinary tract calculi in children.It is suitable for the stones in the pelvis and calyceal where the rigid ureteroscopy could not reach.