INTRODUCTION AND OBJECTIVES: Percutaneous neph- rolithometry (PCNL) may be associated with signi fi cant ionizing radia- tion exposure for patients and operating room personnel. We conducted a multicenter study to assess the predictors of increased radiation exposure during PCNL. METHODS: We performed a multicenter retrospective review of patients undergoing PCNL. Patient demographics, stone characteristics and perioperative data including fl uoroscopy time were recorded and analyzed. Preoperative CT images were reviewed and S.T.O.N.E. score was assigned to each patient. RESULTS: A total of 1700 patients were included in the anal- ysis. The mean age and body mass index (BMI) were 63.4  15 and 31.2  7kg/m2 respectively. Mean stone size and mean overall neph- rolithometry score was 3.7 and 8.3 (SD ¼ 2.2) respectively. Overall sin- gle procedure stone-free rate was 72%. In logistic regression analysis, stone size (p < 0.01), number of calyces (p < 0.01), BMI (p < 0.01), longer procedure time (p < 0.01), skin-to-stone distance (p < 0.01) and S.T.O.N.E. score (p < 0.01) were signi fi cantly associated with increased fl uoroscopy time. Stone-free patients had statistically signi fi cant lower radiation exposure compared to patients with residual stones (8.7 vs. 16.9 minutes, respectively; P < 0.001). On average, for each increase in one unit of S.T.O.N.E score, there was additional 1.5 minutes of fl uo- roscopy time. In risk strati fi cation, low-risk patients with 5-7 scores had 7.8 minutes of fl uoroscopy time, compared to medium-risk (8-10) and high-risk patients (11-13) who had 12.8 and 15.8 minutes of fl uoroscopy time, respectively (p ¼ 0.001). CONCLUSIONS: In this multicenter study, patients with greater stone size, BMI, number of calyces, skin-to-stone distance and S.T.O.N.E. nephrolithometry were associated with increased radiation exposure during PCNL. Alternative modi fi ed pulsed fl uoroscopy tech- niques or radiation-free imaging modalities such as ultrasound should be used in order to reduce the amount of ionizing radiation during the PCNL procedures