Mannitol therapy is widely used for reducing brain edema, and ischemic brain swelling. However, mannitol at clinical concentrations induces apoptosis in endothelial cells. Because apoptosis may be a pathogenic mechanism in vascular injury, antiapoptotic agents may have a protective role in mannitol-induced apoptosis. In this study, we examined whether adrenomedullin (AM) prevents mannitol-induced apoptosis and also evaluated the associated signaling pathway of AM in human umbilical vein endothelial cells. AM prevented mannitol-induced apoptosis in a dose-dependent manner. Pretreatment with wortmannin blocked the AM-induced antiapoptotic effect. AM stimulated Akt at Ser473, and wortmannin inhibited the AM-induced Akt phosphorylation. These findings indicate that phosphatidylinositol 3'-kinase/Akt pathway transmits the survival signal from AM. The potency of antiapoptotic effect of AM is stronger than that of vascular endothelial growth factor and angiopoietin-1 in mannitol-induced apoptosis. AM can have a protective role not only in umbilical vein, but also in pulmonary, coronary, and aortic endothelial cells. These findings indicate that AM has a potent protective role in mannitol-induced apoptosis, through phosphatidylinositol 3'-kinase/Akt pathway. Therefore, pretreatment with AM might help to maintain normal endothelial integrity during systemic mannitol therapy.