Idiopathic antenatal hydronephrosis (IAHN), defined as antenatal hydronephrosis not associated with other morphologic renal tract abnormalities, is the most common abnormality detected by antenatal ultrasound. We performed a systematic review and meta-analysis to determine the outcome of IAHN. We screened three databases and bibliographies to identify English-language original peer-reviewed papers that reported serial postnatal renal ultrasonography in children with IAHN. Patients who stabilized and/or improved were extracted and pooled according to the individual grading systems used by each study. A systematic analysis of data extracted from 25 articles revealed overall resolution of pelviectasis in milder cases of IAHN (Society of Fetal Urology [SFU] grade 1-2; anterior posterior pelvic diameter [APPD]<12 mm). In contrast, IAHN of higher severity (grades 3-4; APPD>12 mm) resolved with a lower frequency. Meta-analysis of data extracted from seven papers showed stabilization of pelviectasis in 98% of patients with grades 1-2 (95% confidence interval [CI] 0.93-1.0; p =0.0008) and in 51% of patients with grades 3-4 (95% CI 0.34-0.68; p <0.00001). Grades 1-2 pelviectasis was five times more likely to stabilize than grades 3-4 pelviectasis (odds ratio [OR] 4.69; 95% CI 1.73-12.76; p =0.002). We conclude that in patients with IAHN and lesser degrees of pelvic dilatation, pelvic diameter decreases to the normal range or does not worsen with the vast majority of patients. Further studies are needed to define outcomes, particularly in more severe forms of IAHN.