A retrospective cohort study was conducted on 115 preterm neonates who underwent patent ductus arteriosus ligation after failure of medical treatment, at Chiang Mai University Hospital between January 2003 and December 2010. Two groups were defined: an early surgery group (39.1%) treated with surgical closure within 21 days, and a late surgery group (60.9%) operated on >21 days after birth. There were significant differences between the 2 groups in terms of weight at surgery, birth weight, duration of intubation, ductal size, and preoperative indomethacin usage. The early surgery group had a longer intensive care unit stay, but differences in postoperative chronic lung disease, ductal size, and surfactant use were not significant. Babies with a low birth weight (<1,500 g) and those with a higher ductal gradient were more susceptible to chronic lung disease, but multivariate analysis showed no difference between early and late surgery patients in either early extubation or chronic lung disease. It was concluded that early patent ductus arteriosus ligation did not show more benefits than late surgery.