Abstract Purpose: To examine the effects of antibiotic prophylaxis on postoperative infection rate in patients with negative urine cultures undergoing ureteroscopy (URS). Patients and Methods: Using the Clinical Research Office of the Endourological Society (CROES) URS Global Study database, patients with a negative baseline urine culture undergoing URS for ureteral stones (n=1141) or kidney stones (n=184) not receiving antibiotic prophylaxis were matched with those who were predefined by risk factors, including gender, American Society of Anesthesiologists (ASA) score, and ureteral stent placement. Patient characteristics, operative data, and postoperative outcomes, including the development of urinary tract infection (UTI) and fever, in the two groups were compared. Results: Antibiotic prophylaxis use differed widely across participating countries (13%-100%). Differences were found between patients who did or did not receive antibiotic prophylaxis regarding the frequency of anticoagulation medication, previous treatment with URS, stone burden, previous presence of kidney stones, duration of current URS, and complications post-URS. The prevalence of fever and UTI was low (≤2.2%) and similar in both groups. Factors predictive of postoperative UTI or fever were female gender, Crohn's and cardiovascular disease, a high stone burden, and an ASA score of II or higher. Conclusions: In patients with a negative baseline urine culture undergoing URS for ureteral or renal stones, rates of postoperative UTI and fever were not reduced by preoperative antibiotic prophylaxis. Female gender and a high ASA score were specific risk factors for postoperative infection in this patient group.