Objective To investigate cerebrovascular reserve(CVR) changes after carotid angioplasty and stenting(CAS) in patients with diabetes mellitus(DM). Methods Fifty-five patients with asymptomatic carotid artery stenosis(≥70%) or symptomatic carotid artery stenosis(≥50%) were divided into DM group(n=16) and non-DM group(n=39). Hypercapnia was induced by inhaling a mixture gas of 5% CO2 and 95% O2, and the changes of blood velocity were recorded in both sides of middle cerebral artery(MCA) before and after CAS. CVR was compared between the two groups before and after CAS. Results CVR was significantly lower in DM group than in non-DM group preoperatively [(8.43±12.80)% vs (29.65±18.23)%, P＜0.05]. After CAS, there was no significant difference in CVR between the two groups[(36.46±15.32)% vs (45.68±13.17)%, P＞0.05]. After CAS, CVR was significantly increased on both the stenosis side [(18.43±12.80)% vs (36.46±15.32)%, P＜0.05] and contralateral side in DM group[(30.40±16.48)% vs (38.28±15.00)%, P＜0.05]. In both two groups, CVR was improved more significantly in stenosis side than in the contralateral side(P＜0.01; P＜0.05). Conclusions DM worsens CVR of the patients with severe carotid artery stenosis. The DM patients and non-DM patients benefit equally from CAS. So, DM can not be a predictive factor for stent placement.