Combined surgery consisting of endoscopic trans-sphenoidal surgery (ETSS) and transcranial (TC) surgery for giant pituitary adenomas (GPAs) has been recommended to prevent lethal postoperative apoplexy. Based on our experience, we attempt to rationalize the indications for such surgery. We report the magnetic resonance (MR) characteristics of the tumor and the outcomes in patients with GPAs who underwent ETSS only and combined surgery. Total tumor volume (TTV), tumor extension volume (TEV), and suprasellar extension of tumor (SET) were calculated based on the lines drawn on MR images and compared between those who underwent ETSS only and those who underwent combined surgery. Of 80 patients with GPAs, eight (10%) underwent combined surgery (seven in the same sitting and one had staged surgery). All eight patients (100%) who underwent combined surgery had tumors with multilobulations, extensions, and encasement of the vessels in the circle of Willis (COW). Of 72 patients who underwent ETSS alone, 21 (29.1%) had a multilobulated tumor, 26 (36.2%) tumors had anterior/lateral extensions, and 12 (16.6%) had encasement of the COW. The mean TTV, TEV, and SET for the combined surgery group were significantly higher than those in the ETSS group. None of the patients who underwent combined surgery suffered postoperative residual tumor apoplexy. Patients with GPAs in whom there are significant lateral intradural or subfrontal tumor extensions should be considered for combined surgery at the same sitting to avoid devastating postoperative apoplexy in the residual tumor, which can occur when ETSS alone is performed.