Observation is a conservative management option in infants with nonrefluxing hydronephrosis, primary nonrefluxing megaureter and ureterocele diagnosed postnatally following antenatal detection of hydronephrosis. Antibiotic prophylaxis might be a sensible regimen under these circumstances to prevent UTI in this population who are potentially at increased risk. However, studies examining the efficacy of prophylactic antibiotics are sparse in this setting. For each condition, prophylactic policies seem extremely variable, and UTI rates vary widely with comparable rates reported between patients followed on and off antibiotics. Overall, antibiotic prophylaxis seems unnecessary in patients with isolated low-grade hydronephrosis. Patients with high-grade nonrefluxing hydronephrosis seem at increased risk of UTI, with risk further increasing in patients with associated ureteral dilatation (hydroureteronephrosis) irrespective of the presence of a ureterocele. Obstruction might be an additional independent risk factor, but the diagnosis of obstruction is often possible only in retrospect. The data available suggest that infants are the most at risk of UTI during the first 6 months of life, particularly if they undergo catheterization during workup examinations. Thus, antibiotic prophylaxis might be prudent during the first 6-12 months of life in patients with high-grade hydronephrosis and hydroureteronephrosis with or without ureterocele, and particularly before completion of the diagnostic workup. Paediatric urologists are urged to embark on controlled trials to compare patients followed with and without antibiotic prophylaxis.