Patients with asymptomatic unruptured intracranial aneurysms (UIA) in the anterior circulation underwent diffusion- and T2-weighted imaging (DWI and T2WI, respectively) 2 days before and 4-5 days after microsurgical clipping. Newly detected lesions on DWI and T2WI after treatment were the primary endpoints of this study. Between 2011 and 2018, 487 patients underwent microsurgical clipping in 512 consecutive procedures for 581 aneurysms. New ischemic lesions were detected on DWI in 48 patients (9.4%): in the territory of arteries adjacent to the aneurysm in 20 patients, in the territory distal to the aneurysm in 20, and in the territory unrelated to the aneurysm in 8. Ischemia of the perforating arteries was observed in 29 patients (5.7%). Size (＞7 mm), the use of temporary occlusion, and no motor-evoked potential (MEP) change were possible risk factors for new ischemic lesions but were not statistically significant. Four (8.3%) of the 48 new ischemic lesions were temporarily symptomatic but fully recovered 6 months after surgery. T2WI detected 144 new lesions in 127 (24.4%) patients, with 80 being approach-related, 14 venous congestion, 48 ischemic, and 2 hemorrhagic. The volume of affected brain ranged from 0.03 to 18.5 mL (mean 1.54). Approach-related, venous congestion, and hemorrhagic lesions were all asymptomatic. All 48 ischemic lesions detected on DWI were also detected on T2WI. Sixty-four of 72 (88.9%) approach-related and 8 of 11 (72.7%) venous congestion lesions were not detected on magnetic resonance imaging (MRI) at the 1-year follow-up. In conclusion, early postoperative MRI is useful for assessing the quality of microsurgery by objectively reflecting surgical invasiveness with high sensitivity.