The X chromosome-linked inhibitor-of-apoptosis protein (XIAP) contributes to apoptosis regulation after a variety of cell death stimuli. XIAP inhibits the caspase reaction via binding to caspases, and is inhibited via binding to the second mitochondria-derived activator of caspase (Smac)/DIABLO to tightly control apoptotic cell death. However, the interaction among XIAP, Smac/DIABLO, and caspases after in vivo cerebral ischemia is not well known. To clarify this issue, the authors examined time-dependent expression and interaction among XIAP, Smac/DIABLO, and activated caspase-9 by immunohistochemistry, Western blot analysis, and immunoprecipitation using an in vivo transient focal cerebral ischemia model. To examine the relationship of the XIAP pathway to the caspase cascade, a pan-caspase inhibitor was administered. XIAP increased concurrently with the release of Smac/DIABLO and the appearance of activated caspase-9 during the early period after reperfusion injury. The bindings of XIAP to Smac/DIABLO and to caspase-9 and the binding of Smac/DIABLO to caspase-9 reached a peak simultaneously after transient focal cerebral ischemia. Neither XIAP nor Smac/DIABLO expression was affected by caspase inhibition. These results suggest that the XIAP pathway was activated upstream of the caspase cascade and that interaction among XIAP, Smac/DIABLO, and caspase-9 plays an important role in the regulation of apoptotic neuronal cell death after transient focal cerebral ischemia.