['To evaluate the impact of body mass index (BMI) on perioperative outcomes and radiation exposure for ultrasound (US)-guided percutaneous nephrolithotomy (PCNL).', 'Data were prospectively collected for consecutive patients who underwent PCNL at the University of California, San Francisco, from July 2013 to November 2015. Patients were divided into 3 groups according to their BMI:\u2009<25 (normal weight), 25-29.9 (overweight), and >30 (obese) kg/m', 'One hundred thirty-five patients were enrolled; 93 cases were performed under US and 42 under fluoroscopic guidance. US successfully guided renal access in 76.9% of normal weight, 79.0% of overweight, and 45.7% of obese patients (P\u2009<\u2009.05). Mean fluoroscopic screening time and radiation exposure dose were reduced for US compared to fluoroscopy cases across all BMI categories (P\u2009<\u2009.05). As BMI increased, radiation exposure dose rose disproportionately faster compared to screening time (P\u2009<\u2009.001). No significant differences among the BMI groups were found with regard to complication rate, hospital stay, and stone-free status.'] Does ultrasound Guidance to Assist Percutaneous Nephrolithotomy reduce Radiation Exposure in Obese Patients?