The aim of the study to compare outcomes of flexible ureterorenoscopy in patients with different body mass index (BMI) scores and to explore whether the BMI has an effect on outcomes of RIRS. Five hundred and two patients who underwent flexible URS in 3 centers between 2008 and 2012 for the management of single upper urinary tract calculi were retrospectively reviewed. Patients were categorized as normal weight BMI 18.5 to 24.99 kg/m(2), overweight 25 to 29.99 kg/m(2), obese 30 to 39.99 kg/m(2) and morbid obese > 40 kg/m(2).The groups were assessed in terms of demographic parameters including age, gender, stone size, intraoperative and postoperative variables. The mean patient age was 41.3 +/- A 15.51 (18-81) years and with an average BMI 26.68 +/- A 5.2 kg/m(2) (16.64-55.15 kg/mA(2)). Of the patients, 43.2 % had normal weight (NW), 32.2 % were overweight (OW), 21.9 % were obese (O) and 2.5 % were morbidly obese (MO). Stone-free rates after single procedure in NW, OW, O, MO groups were 60.8, 61.7, 73.6, 61.5 %, respectively (p = 0.079). Overall targeted stone-free rates were also similar in four groups (88.9, 90.1, 93.6, 90.4 %, p = 0.586). There were no statistically significant differences in the frequency of complications and mean hospitalization time among the groups (p > 0.05). In conclusion, this study demonstrated that flexible URS is a valuable option for the treatment of kidney stone in both obese and non-obese patients. BMI did not influence the postoperative outcomes.