Paclitaxel induces apoptosis via caspase-3 activation in human osteogenic sarcoma cells (U-2 OS). Paclitaxel has been found to exhibit cytotoxic and antitumor activity. There is little information regarding the mechanisms of apoptotic-inducing effect of paclitaxel on human osteogenic sarcoma U-2 OS cells. Several key regulatory proteins are involved in the initiation of apoptosis. Caspase-3 plays a direct role in proteolytic cleavage of cellular proteins responsible for progression to apoptosis. We examined the effect of paclitaxel on the cell cycle arrest and apoptosis in U-2 OS cells using flow cytometric analysis and Western blotting. We also measured the inhibition of paclitaxel-induced apoptosis and the caspase-3 activity by the broad-spectrum caspase inhibitor z-VAD-fmk on U-2 OS cells. The increased levels of casapse-3 were also confirmed by cDNA microarray. Our observations were: (1) paclitaxel treatment resulted in G2/M-cycle arrest in U-2 OS cells; (2) time and dose dependent apoptosis of U-2 OS cells was induced by paclitaxel; (3) in U-2 OS cells, z-VAD-fmk blocked the paclitaxel-induced apoptosis and caspase-3 activation. These results suggest that paclitaxel-induced G2/M-cycle arrest of the G2/M phase and apoptosis via a caspase-3 pathway in U-2 OS cells.