Background: The objective of this study is to establish a potential correlation between postoperative improvement in upper eyelid ptosis, measured by the marginal reflex distance 1 (MRD1), and the immediate postoperative degrees of lagophthalmos (IPDL). The established correlation is then used to assess whether IPDL can be used as a reliable indicator of successful treatment of eyelid ptosis. Methods: This retrospective study involved 19 patients, aged 1 to 11, with a total of 28 eyes affected by ptosis, who underwent surgery under general anesthesia at Seoul National University Bundang Hospital from January 2018 to December 2022. We monitored the MRD1 of the patients for over six months postoperatively and measured the IPDL. Results: After ptosis correction surgery, no statistically significant correlation was observed between the improvement in MRD1 and IPDL. Furthermore, the degree of postoperative eyelid ptosis improvement was found to vary and was not consistently sustained, attributable to a range of factors. Conclusions: Our study did not establish a statistically significant correlation between IPDL and ptosis improvement as measured by MRD1. Further research is needed to draw definitive conclusions about their correlation.