To assess effectiveness of flexible ureteroscopy (fURS) in patients not with simple but with multiple unilateral renal stones. A database analysis was performed to identify patients with multiple unilateral intrarenal stones. Patients were treated using an 8.8F flexible ureteroscope. Patients were considered stone free if no residual stones were seen endoscopically and radiographically after the procedure. The efficiency and the safety of the procedures were analyzed. Between 2006 and 2008, 1054 ureteroscopic procedures were performed in our department; 125 patients met the inclusion criteria. Holmium:yttrium-aluminum-garnet laser lithotripsy was necessary in 41 (32.8%) patients. The mean stone size was 11.93 ± 8.2 mm, with a mean stone burden of 83.7 ± 67.66 mm(2). The mean number of stones was 3.59 ± 3.57. Twenty-six (20.8%) patients had a stone burden ≥ 100 mm(2), and 99 (79.2%) a stone burden <100 mm(2). The overall stone-free rate after a single procedure of fURS was 74.4%. In the remaining 32 patients with residual stones, the mean stone burden dropped from 123.24 ± 84.36 mm(2) to 56.28 ± 52.53 mm(2). The stone-free rate in patients with a stone burden ≥ 100 mm(2) was 65.4% and 79.5% in patients with a stone burden < 100 mm(2), respectively. Complications have been recorded in seven (5.6%) patients, including urinary tract infection in four, ureteral perforation in one, and hematuria in two patients. No major complications occurred. All patients were treated conservatively. fURS is an effective treatment option in patients with multiple unilateral renal stones. Single procedure stone-free rates are high with a low rate of only minor complications.