Objectives: To compare the stone free rate and treatment success rate between a diuretic group of patients undergoing extracorporeal shock wave lithotripsy (ESWL) and a control placebo group (normal saline solution). Materials and Methods: One hundred and ninety-four patients with solitary renal calculi or ureteric calculi size of 5 mm or over were prospectively randomized into 2 groups. Ninety-seven patients in the first group (diuretic group) underwent ESWL after intravenous injection of furosemide 40 mg, and 97 patients in the second (control) group received normal saline solution 4 ml instead of furosemide prior to ESWL. The treatment protocol included 3,000 shockwaves per patient in each session with the energy beginning at 8 and progressing up to 15 kilovolts. A maximum of 3 ESWL sessions were permitted per patient. The primary outcome was stone free rate, and the secondary outcome was treatment success rate at 3 months after the first ESWL treatment. Results: The stone free rate was 48.5% compared to 50.5% for diuretic group and control group respectively and the treatment success rate was 81.4% compared to 64.9%. The difference in stone free rate was not statistically significantly different (p = 0.87), however the treatment success was, p = 0.01. Conclusion: A combination of diuretic therapy followed by ESWL improves the treatment success rate compared with standard ESWL therapy alone.