The oxygen reserve index (ORi™) is a parameter used for the noninvasive evaluation of arterial partial pressure of oxygen (PaO), specifically in the 100-200 mmHg range. We aimed to report on the impact of indocyanine green (ICG) on the ORi™.In this study, we retrospectively examined patients who underwent neurosurgery between April and July 2019 and assessed the impact of ICG on ORi™. We excluded patients who did not use ICG or who were not examined for ORi™. The dose and timing of ICG administration were determined by a neurosurgeon. The changes in ORi™ were measured for up to 30 min.We analyzed ten patients and found that the ORi™ increased to 1.00 in all of them. The median time for ORi ™ to rise to 1.00 after ICG administration was 2 min (range 1-4). After rising to 1.00, ORi ™ decreased and took 27 min to return to the pre-dose value.It is important to consider the initial rapid increase and subsequent slow decrease in ORi™ when using ICG during surgery.