Hypoxia/re-oxygenation injury induces apoptosis in renal tubule cells, but its underlying molecular pathways are not fully elucidated. Activation of caspase-2 has recently been proposed as a novel mechanism of apoptosis in fibroblasts. In this study we examined whether hypoxia/re-oxygenation injury induces apoptosis in proximal tubule cells by activation of caspase-2. Porcine proximal tubule (LLC-PK1) cells were subjected to hypoxia/re-oxygenation injury in the presence or absence of caspase inhibitors. Apoptosis was detected by DNA laddering, flow cytometry, and immunocytochemistry for Bax and cytochrome c. The activity of caspases-2, 8 and 9 was measured. Apoptosis was evident after hypoxia/re-oxygenation and was best prevented by pretreatment with caspase-2 inhibitor. Hypoxia/re-oxygenation resulted in a dramatic increase in caspase-2 activity (32-fold, in comparison with a 16-fold increase in caspase-8 activity and a tenfold increase in caspase-9 activity). Immunocytochemistry revealed Bax activation and translocation to mitochondria and cytochrome c release into the cytosol following hypoxia/re-oxygenation, both of which were significantly suppressed by pretreatment with caspase-2 inhibitor. These results indicate that hypoxia/re-oxygenation injury in cultured proximal tubule cells induced apoptosis by activation of caspase-2, which is required for the mitochondrial translocation of Bax.