Abstract Objectives To assess the outcome of newborns with mild isolated antenatal hydronephrosis (MIAHN) managed with neither antibiotic prophylaxis nor voiding cystourethrography (VCUG). Material and methods Inclusion criteria: anterior-posterior pelvic diameter 5–15 mm at third trimester of gestation, confirmed by first postnatal ultrasound. Exclusion criteria: pelvic diameter > 15 mm, calyectasis, hydroureteronephrosis, renal or bladder abnormalities. Clinical and ultrasound follow-up was performed. Parents were familiarized with urinary tract infection (UTI) signs. If UTI was confirmed, VCUG was performed. Hydronephrosis outcome was assessed as intrauterine resolution, total or partial resolution, stability or progression. Results MIAHN was detected in 193 newborns (109 unilateral, 84 bilateral; 277 renal units); 23 (12%) had UTI and 2 of them showed low-grade reflux. After a mean follow-up of 15 months, 91 renal units showed intrauterine resolution (33%), 111 (40%) total resolution, 20 (7%) partial resolution, 52 (19%) stability and 3 (1%) progression. Conclusion Total resolution of hydronephrosis was observed in 73% of renal units during the first year. Routine antibiotic prophylaxis and VCUG might not be necessary in all infants with MIAHN, clinical and ultrasound follow-up being advisable during the first year of life.