In infants with ureteropelvic junction obstruction (UPJO), the risk of urinary tract infection (UTI) is unknown, and there is a lack of prospective studies showing definitive evidence regarding the benefits and necessity of antibiotic prophylaxis. The aim of this study was to assess the risk of UTI in infants with UPJO and to determine whether the risk varies according to the degree of hydronephrosis. Infants with hydronephrosis detected prenatally or within the postnatal 28th day and who had no previous history of UTI were followed prospectively without antibacterial prophylaxis. Imaging studies were performed according to our Pediatric Uro-Nephrology Study Group protocol. Dimercaptosuccinate (DMSA) scintigraphy was performed in all infants at the end of 1 year of follow-up. Eighty-four infants (56 boys, 28 girls) were included in the study. The distribution of patients in each hydronephrosis grading group was incidentally similar. Within a median follow-up period of 18 (12-24) months, none of the patients had UTI. Furthermore, no pyelonephritic scar was found on DMSA scans in any patient. We conclude that prophylactic antibiotic usage is not indicated in infants with UPJO, regardless of the severity of hydronephrosis, as the risk of UTI is minimal in this population.