It was reported that cilostazol (CLZ) suppressed disruption of the microvasculature in ischemic areas. In this study, we have designed novel injection formulations containing CLZ nanoparticles using 0.5% methylcellulose, 0.2% docusate sodium salt, and mill methods (CLZ(nano) dispersion; particle size 81 +/- 59 nm, mean +/- S.D.), and investigated their toxicity and usefulness in a cerebral ischemia/reperfusion-induced injury model (MCAO/reperfusion mice). The pharmacokinetics of injections of CLZ(nano) dispersions is similar to that of CLZ solutions prepared with 2-hydroxypropyl--cyclodextrin, and no changes in the rate of hemolysis of rabbit red blood cells, a model of cell injury, were observed with CLZ(nano) dispersions. In addition, the intravenous injection of 0.6 mg/kg CLZ(nano) dispersions does not affect the blood pressure and blood flow, and the 0.6 mg/kg CLZ(nano) dispersions ameliorate neurological deficits and ischemic stroke in MCAO/reperfusion mice. It is possible that the CLZ(nano) dispersions will provide effective therapy for ischemic stroke patients, and that injection preparations of lipophilic drugs containing drug nanoparticles expand their therapeutic usage.