Background: Prone positioning is common during spine procedures. Because the optic nerve sheath is an extension of dura mater, optic nerve sheath diameter (ONSD) reflects changes in intracranial pressure (ICP). Measurement of ONSD is recommended as a rapid, reliable, reproducible and non-invasive technique for the evaluation of ICP. In this study, our primary objective was to determine prone position effect on ICP in patients undergoing lumbar surgery using ultrasonographic measurements of ONSD, and secondly to evaluate factors that might have impact on ONSD. Material and Methods: Our study is a single centered prospective study. Written informed consent was obtained from all patients, and study protocol was approved by institutional committee, ensuring ethical guidelines of Declaration of Helsinki. Patients in ASA I-III risk group, aged between 18-80 years, who would undergo lumbar surgery under elective conditions were included. Standard anesthesia was administered with bispectral index monitoring. Optic ultrasonography was assessed in B mode via linear probe in both transverse and sagittal planes. Control measurements (T0) were taken before intubation, and second measurements in supine position after intubation (T1). At the end of the surgery, last measurements were taken in prone position (T2) and then in supine position (T3) before patient was awakened. The percentage of change in ONSDs were calculated. In the comparison of two independent groups for continuous variables, Student's t-test was used, correlation coefficients and statistical significance between normally distributed variables were calculated using Pearson's test. Results: Of the 84 patients, 49 (58.3%) were female and 35 (41.7%) were male. The mean age of the patients was 52.4±14.3 years, height was 168.6±8.6 cm and body weight was 79.4±12.7 kg. A total of 39 (46.4%) patients had comorbid disease. Mean ONSD at T1, T2, and T3 were similar and were found to be significantly higher than T0 measurements (p<0.001). There was no statistically significant correlation between ONSD in the prone position with age, obesity, presence of comorbid disease, hemodynamic variables, duration of anesthesia and amount of fluid administered during the operation. Conclusion: Ocular ultrasonography, a non-invasive, bedside technique, plays a significant role in the identification of elevated intracranial pressure in prone positioning.