Objectives To determine a follow‐up plan for mild congenital hydronephrosis in Japanese individuals. Methods Neonates at Kansai Medical University Hospital (Hirakata, Osaka, Japan) who were diagnosed with mild congenital hydronephrosis – defined as a Society for Fetal Urology grade 1 or 2 – at 1‐month old by abdominal ultrasonography between 2014 and 2016 were enrolled. These patients were encouraged to undergo repeated abdominal ultrasonography for 2 years every 3 months to investigate the course of congenital hydronephrosis. Results Among 1009 neonates, congenital hydronephrosis was detected in 118 affected renal units of 100 (9.9%) patients. According to the definition of the Society for Fetal Urology, 118 affected renal units were graded as grade 1 in 87 (74%), grade 2 in 30 (25%), grade 3 in one (1%) and grade 4 in 0 units. Among them, 117 affected renal units of mild congenital hydronephrosis comprising grades 1 and 2 were subjected to ultrasonographic evaluation to monitor the natural course. The rates of resolution at 7, 10, 13, 16, 19, 22 and 25 months after birth in Society for Fetal Urology grades 1 and 2 cases were 60% and 8%, 77% and 19%, 90% and 32%, 92% and 40%, 95% and 52%, 96% and 56%, and 99% and 60%, respectively. Conclusions Grade 1 congenital hydronephrosis does not need to be followed up, because it mostly shows spontaneous resolution by 2 years of follow up without any complications. However, ultrasonographic examinations at 1‐year intervals for grade 2 congenital hydronephrosis are recommended to determine the subsequent follow‐up plan of patients.

Congenital hydronephrosis (CH) have the highest incidence among congenital malformations that are found in the fetal or neonatal period. As the prognosis of the CH is not always benign, neonates with CH need to be followed up. Although repeated renal ultrasonographic examination is recommended for the follow-up of CH, no standardized strategy is currently available for mild CH due to lack of large cohort surveys on their natural course. Therefore, this study was conducted to determine a follow-up plan for mild CH in Japanese individuals. Among 1009 neonates, CH was detected in 118 affected renal units of 100 (9.9%) patients. According to the definition of the Society for Fetal Urology, 118 affected renal units were graded as grade 1 in 87 (74%), grade 2 in 30 (25%), grade 3 in one (1%) and grade 4 in 0 units. The grade 1 CH was improved 90% and 99%, by 13 and 25 months of age. The grade 2 CH was improved 32% and 60%, by 13 and 25 months of age. In conclusion, Grade 1 CH does not need to be followed up, because it mostly shows spontaneous resolution by 2 years of follow up without any complications. However, ultrasonographic examinations at 1-year intervals for grade 2 CHs are recommended to determine the subsequent follow-up plan of patients.