Abstract Objective: To evaluate the impact of body mass index (BMI) on the outcomes of percutaneous nephrolithotomy (PCNL) in the flank-free modified supine position. Patients and Methods: A prospective study was carried out in the urology department during the period from May 2015 to October 2019 on 464 patients admitted for PCNL. The patients were divided into 4 matched groups according to their BMI: group A, normal weight with 18.5 ≤ BMI <25 kg/m2; group B, overweight with 25 ≤ BMI <30 kg/m2; group C, obese with 30 ≤ BMI <40 kg/m2; and group D, morbid obesity with BMI ≥40 kg/m2. All operative data as well as postoperative outcomes are recorded and compared to each other. Results: The 4 studied groups were matched regarding age. The comorbidities were slightly higher in groups C and D. The operative time and fluoroscopy time were slightly high in obese and morbid obese groups but with no significant difference. The rate of complications either major or minor was comparable in all groups. No significant difference was seen among all groups regarding hemoglobin loss, stone-free rate, hospital stay, and need for auxiliary procedures. Conclusions: The outcome of PCNL in flank-free modified supine position is not affected by changes in BMI. The procedure can be performed in obese and morbid obese patients safely with results similar to and comparable to nonobese patients.