Background: Several clinical studies have indicated the efficacy of cilostazol, a selective inhibitor of phosphodiesterase 3, in preventing cerebral vasospasm after aneurysmal subarachnoid hemorrhage (SAH). They were not double-blinded trial resulting in disunited results on assessment of end points among the studies. The randomized, double-blind, placebo-controlled study was performed to assess the effectiveness of cilostazol on cerebral vasospasm. Methods: Patients with aneurysmal SAH admitted within 24 h after the ictus who met the following criteria were enrolled in this study: SAH on CT scan was diffuse thick, diffuse thin, or local thick, Hunt and Hess score was less than 4, administration of cilostazol or placebo could be started within 48 h of SAH. Patients were randomly allocated to placebo or cilostazol after repair of a ruptured saccular aneurysm by aneurysmal neck clipping or endovascular coiling, and the administration of cilostazol or placebo was continued up to 14 days after initiation of treatment. The primary end point was the occurrence of symptomatic vasospasm (sVS), and secondary end points were angiographic vasospasm (aVS) evaluated on digital subtraction angiography, vasospasm-related new cerebral infarction evaluated on CT scan or MRI, and clinical outcome at 3 months of SAH as assessed by Glasgow Outcome Scale, in which poor outcome was defined as severe disability, vegetative state, and death. All end points were evaluated with blinded assessment. Results: One hundred forty eight patients were randomly allocated to the cilostazol group (n = 74) or the control group (n = 74). The occurrence of sVS was significantly lower in the cilostazol group than in the control group (10.8 vs. 24.3%, p = 0.031), and multiple logistic analysis showed that cilostazol use was an independent factor reducing sVS (OR 0.293, 95% CI 0.099-0.568, p = 0.027). The incidence of aVS and vasospasm-related cerebral infarction were not significantly different between the groups. Poor outcome was significantly lower in the cilostazol group than in the control group (5.4 vs. 17.6%, p = 0.011), and multiple logistic analyses demonstrated that cilostazol use was an independent factor that reduced the incidence of poor outcome (OR 0.221, 95% CI 0.054-0.903, p = 0.035). Severe adverse events due to cilostazol administration did not occur during the study period. Conclusions: Cilostazol administration is effective in preventing sVS and improving outcomes without severe adverse events. A larger-scale study including more cases was necessary to confirm this efficacy of cilostazol.