Purpose <p>Intracranial vessel wall enhancement (VWE) on MR imaging was associated with progression and poor outcome of moyamoya disease (MMD). However, it remains controversial how the signal intensity (SI) of VWE fluctuates in the perioperative period. The purpose of this study was to investigate the change in the SI of VWE between before and after direct bypass surgery for MMD.</p> Methods <p>Subjects undergoing direct bypass surgery for MMD were prospectively recruited. All subjects underwent pre/post operative 3T-MR imaging including pre/post contrast vessel wall imaging in the terminal of internal carotid artery (ICA) and ultrasound evaluation of the superficial temporal artery (STA). The VWE-to-pituitary stalk ratio (CRstalk) was calculated as the contrast ratio of the maximal SI of VWE in the ipsilateral ICA terminal over the maximal pituitary stalk enhancement. Postoperative changes in CRstalk were examined, and the relationship between the delta difference of the CRstalk and that of each value of the STA between before and after the surgery was analyzed.</p> Results <p>A total of consecutive 12 patients with 16 hemispheres were enrolled and all bypass procedures were patent. The postoperative CRstalk was significantly lower than the preoperative CRstalk (0.45 ± 0.21 versus 0.59 ± 0.26,&#xa0;<i>p</i> &lt; 0.01). Additionally, there was a significant negative correlation between delta CRstalk and delta blood volume in the STA (R2 = 0.42,&#xa0;<i>p</i> &lt; 0.01).</p> Conclusions <p>The SI of VWE in the ipsilateral ICA terminal was significantly decreased after direct bypass surgery for MMD. Direct bypass surgery could improve the cerebral perfusion and may attenuate the progression as well as activity of MMD.</p>