Purpose: Conservative management of patent ductus arteriosus ( PDA) in preterm infants remains controversial. However, there is a tendency to lean towards a conservative approach in neonatal intensive care units (NICUs) because of scant evidence of benefits of mandatory PDA closure and also because of adverse effects which may accompany the treatment. There are few studies which have investigated the clinical nature of ductus arteriosus when untreated. We applied a PDA management protocol with a conservative, non-interventional approach in preterm infants including extremely premature infants (EPIs). The purpose of this study is to investigate the natural course of untreated PDA and spontaneous closure rate according to different gestational age in EPIs. Methods: We reviewed the medical records of premature infants with a gestational age (GA) of 23-26 weeks born and admitted to Samsung Medical Center during January 2011 – June 2014, the period we adapted conservative, non-interventional approach in our NICU. Demographics, outcome and clinical variables about PDA characteristics were analyzed. Results: Out of a total of 135 infants, 127 (94%) underwent spontaneous closure during hospitalization with a mean age at closure of 45 ± 30 days. Of the 8 infants with open PDA present at hospital discharge, 6 had spontaneous closure within the first 6 months following discharge, and 2 received device closure at the age of 12 and 13 months, respectively. The ages of closure (days) of the infants with spontaneous closure during hospitalization were 69 ± 60 for GA 23 weeks, 53 ± 27 for GA 24 weeks, 41 ± 17 for GA 25 weeks, and 40 ± 15 for GA 26 weeks (p-value of 0.01 in ANOVA). Conclusions: Conservative, non-interventional approach with no use of ibuprofen or surgical ligation for PDA in EPIs successfully led to spontaneous closure during hospitalization for most infants, and for most of the remaining, during the first 6 months after hospital discharge. The time of spontaneous closure was delayed in younger GA infants compared with older GA infants, which suggests that spontaneous closure after birth is regulated developmentally according to gestational age in EPIs.