BACKGROUND: Aspirin has been implicated in the postoperative management of moyamoya disease (MMD) in order to avoid bypass failure and decrease the incidence of subsequent stroke. However, its effect has not been completely determined yet. METHODS: In this study, we retrospectively reviewed data of 184 adult patients (197 hemispheres) presented with ischemic-onset MMD who had undergone direct or combined revascularization in our hospital, to clarify the effect of postoperative aspirin therapy in the management of moyamoya disease. Fifty-nine hemispheres that had been administered with aspirin (100 mg/day) after bypass surgery were defined as the "aspirin group," whereas 138 that hadn't been given aspirin postoperatively were defined as the "control group". RESULTS: Among 197 hemispheres, the mortality rate was 0. The incidence of postoperative newly developed infarction, transient ischemic attack, and hemorrhage were not significantly different between the aspirin and control groups. The patency rate of bypass graft was not significantly different between the groups, either. Notably, more patients experienced major stroke in the control group (9/138) than the aspirin group (1/59), but no statistical difference was found (P > 0.05). In the aspirin group, more patients had improved outcome than the control group (P = 0.04). CONCLUSIONS: Our findings showed that aspirin might not decrease the incidence of postoperative ischemic stroke or increase patency rate of bypass graft, but it does not increase the risk of hemorrhages, either. Also, postoperative aspirin therapy might improve outcome. More studies are needed to provide evidence for postoperative antiplatelet therapy in MMD management