Mannitol has been frequently used for renal protection during surgery. Optic nerve sheath diameter is used to evaluate intracranial pressure noninvasively. Robot-assisted laparoscopic radical prostatectomy can induce an increase in optic nerve sheath diameter due to Trendelenburg position and carbon dioxide insufflation. We therefore aim to evaluate the change in optic nerve sheath diameters before and after mannitol administration in patients who undergo robot assisted laparoscopic radical prostatectomy.